UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 11-K

(Mark One)

Commission file number 1-8993

A.                                   Full title of the plan and the address of the plan, if different from that of the issuer named below:

ONEBEACON INSURANCE SAVINGS PLAN

B.                                     Name of issuer of the securities held pursuant to the plan and the address of its principal executive office:

WHITE MOUNTAINS INSURANCE GROUP, LTD.

80 South Main Street
Hanover, NH 03755
(603) 640-2200

REQUIRED INFORMATION

The following Financial Statements and Schedule for the Plan and a Written Consent of Independent Registered Public Accounting Firm are filed with, and included in, this Report as Exhibits 99(a) and 99(b) hereto, respectively, as detailed below:

99(a)                                              Financial Statements and Schedule for the Plan consisting of:

1.                       Report of Independent Registered Public Accounting Firm;

2.                       Statements of Net Assets Available for Benefits as of December 31, 2004 and 2003;

3.                       Statements of Changes in Net Assets Available for Benefits for the years ended December 31, 2004 and 2003;

4.                       Notes to Financial Statements;

5.                       Schedule of Assets (Held at End of Year) December 31, 2004;

99(b)                                             Consent of Independent Registered Public Accounting Firm

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SIGNATURES

THE PLAN.  Pursuant to the requirements of the Securities Exchange Act of 1934, the Benefits Committee has duly caused this annual report to be signed on its behalf by the undersigned, thereunto duly authorized.

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EXHIBIT INDEX

Exhibit 99(a)

OneBeacon Insurance Savings Plan

Financial Statements and Supplemental Schedule to

Accompany 2004 Form 5500

Annual Report of Employee Benefit Plan

Under ERISA of 1974

For the Years Ended December 31, 2004 and 2003

OneBeacon Insurance Savings Plan

Index of Financial Statements and Supplemental Schedule

*                 Other schedules required by Section 2520.103-10 of the Department of Labor’s Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974 have been omitted because they are not applicable.

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Report of Independent Registered Public Accounting Firm

To the Participants and Administrator of

OneBeacon Insurance Savings Plan

In our opinion, the accompanying statements of net assets available for benefits and the related statements of changes in net assets available for benefits present fairly, in all material respects, the net assets available for benefits of OneBeacon Insurance Savings Plan (the “Plan”) at December 31, 2004 and 2003, and the changes in net assets available for benefits for the years then ended in conformity with accounting principles generally accepted in the United States of America.  These financial statements are the responsibility of the Plan’s management.  Our responsibility is to express an opinion on these financial statements based on our audits.  We conducted our audits of these statements in accordance with the standards of the Public Company Accounting Oversight Board (United States).  Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement.  An audit includes examining, on a test basis, evidence supporting the amounts and disclosures in the financial statements, assessing the accounting principles used and significant estimates made by management, and evaluating the overall financial statement presentation.  We believe that our audits provide a reasonable basis for our opinion.

Our audits were conducted for the purpose of forming an opinion on the basic financial statements taken as a whole.  The supplemental schedule of assets (held at end of year) at December 31, 2004 is presented for the purpose of additional analysis and is not a required part of the basic financial statements but is supplementary information required by the Department of Labor’s Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974.  This supplemental schedule is the responsibility of the Plan’s management.  The supplemental schedule has been subjected to the auditing procedures applied in the audits of the basic financial statements and, in our opinion, is fairly stated in all material respects in relation to the basic financial statements taken as a whole.

PricewaterhouseCoopers LLP

Boston, Massachusetts

June 28, 2005

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OneBeacon Insurance Savings Plan

Statements of Net Assets Available for Benefits

December 31, 2004 and 2003

The accompanying notes are an integral part of these financial statements.

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OneBeacon Insurance Savings Plan

Statements of Changes in Net Assets Available for Benefits

For the years ended December 31, 2004 and 2003

The accompanying notes are an integral part of these financial statements.

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OneBeacon Insurance Savings Plan

Notes to Financial Statements

A.           Description of the Plan

The following description of the OneBeacon Insurance Savings Plan (“Plan”) provides only general information. Participants should refer to the Plan agreement and related amendments for a more complete description of the Plan’s provisions.

General

The Plan is a defined contribution plan covering substantially all employees of OneBeacon Insurance Company (the “Company”). On March 31, 2004, OneBeacon acquired Atlantic Specialty Insurance Company, a subsidiary of Atlantic Mutual Insurance Company (“Atlantic Mutual”), and the renewal rights to Atlantic Mutual’s segmented commercial insurance business, including the unearned premiums on the acquired book. The Plan was amended to allow former Atlantic Mutual employees hired by OneBeacon to participate in the Plan.

The Plan is subject to the provisions of the Employee Retirement Income Security Act of 1974 (“ERISA”).

Eligibility

Employees who complete sixty days of continuous service are eligible to participate in the Plan.  Eligible employees are automatically enrolled in the Plan at a 2 percent contribution rate, unless waived by the employee.

Contributions

Effective January 1, 2003, participants could contribute 40 percent of annual compensation on a pre-tax and/or an after-tax basis.  Participants direct their contributions into various investment options offered by the Plan. The Company contributes 50 percent of the first 6 percent of base compensation that a participant contributes to the Plan.  The matching Company contribution mirrors the employee directed investment options.  Eligible participants who attain age 50 before the end of the Plan year can make catch up contributions to the Plan.  Contributions are subject to IRS limitations.

Employees hired between June 1, 2001 and April 11, 2003 have been provided with the equivalent of two common shares of White Mountains Insurance Group, Ltd. on the first day of the second month of the quarter following one year of service with the Company. In 2004 and 2003, the Company contributed $491,797 and $729,422, respectively, to these employees.  Contributions into the White Mountains Stock Fund can be immediately directed by the participant into another investment option.

Participant Accounts

Each participant’s account is credited with the participant’s contribution and allocations of (a) the Company’s contribution (b) Plan earnings, net of an allocation of investment fees and (c) applicable loan fees. Allocations are based on participant earnings or account balances, as defined. The benefit to which a participant is entitled is the benefit that can be provided from the participant’s vested account.

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Vesting

Participants are vested immediately in their contributions plus actual earnings thereon. Vesting in the Company’s contribution portion of their accounts is based on years of continuous service. A participant is 100 percent vested after three years of credited service.

Forfeitures

Forfeitures are used to reduce future Company contributions. The balances as of December 31, 2004 and 2003, in the forfeiture account were $4,248 and $11,952, respectively. During 2004 and 2003, $562,635 and $428,273, respectively, of forfeited funds were used to offset employer contributions.

Participant Loans

Participants may borrow from their fund accounts a minimum of $500 up to a maximum of $50,000 or 50 percent of their vested account balance, whichever is less. The loans are secured by the balance in the participant’s account and bear interest at the prime rate (5.25% and 4.00% at December 31, 2004 and December 31, 2003, respectively) plus 1 percent as of the beginning of the month in which the loan was made.

Payment of Benefits

On termination of service due to death, disability, or retirement, a participant may elect to receive either a lump-sum amount equal to the value of the participant’s vested interest in his or her account, or annual installments over time. For termination of service for other reasons, a participant may only receive the value of the vested interest in his or her account as a lump-sum distribution.

Expenses

The Company paid the majority of administrative expenses relating to the Plan except for certain investment management fees paid by the Plan to Vanguard of $221,941 and $269,944 in 2004 and 2003, respectively.

Plan Termination

While the Company has not expressed any intent to discontinue its contributions or terminate the Plan, it is free to do so at any time.  In the event the Plan is terminated, the Plan provides that each participant’s balance, inclusive of Company contributions, becomes immediately 100 percent vested and shall be distributed to the participants.

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B.             Investment Options

During the plan years ended December 31, 2004 and 2003, participants were able to allocate their contributions among various registered investment company options, a company stock fund and four specific Plan-sponsored funds comprised of stocks, bonds, government securities and guaranteed investment contracts as follows:

Columbia Mid Cap Value Fund

OneBeacon Equity Fund

OneBeacon Fixed Income Fund

OneBeacon Fully Managed Fund

OneBeacon Insurance Group Stable Value Fund

Vanguard 500 Index Fund Investor Shares

Vanguard Asset Allocation Fund

Vanguard Extended Market Index Fund Investor Shares

Vanguard High-Yield Corporate Fund

Vanguard International Growth Fund

Vanguard Long-Term Corporate Fund Investor Shares

Vanguard Morgan Growth Fund Investor Shares

Vanguard Prime Money Market Fund

Vanguard Short-Term Corporate Fund Investor Shares

Vanguard Small-Cap Index Fund Investor Shares

Vanguard Total International Stock Index Fund

Vanguard U.S. Growth Fund

Vanguard Wellington Fund Investor Shares

Vanguard Windsor Fund

Vanguard Windsor II Fund Investor Shares

White Mountains Stock Fund

C.             Summary of Accounting Policies

The following accounting policies, which conform to accounting principles generally accepted in the United States of America, have been used consistently in the preparation of the Plan’s financial statements and notes to financial statements:

Basis of Accounting

The financial statements of the Plan are prepared under the accrual method of accounting.

Use of Estimates

The preparation of financial statements in conformity with generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of additions and deductions during the reporting period.  Actual results could differ from those estimates.

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Investment Valuation and Income Recognition

The Plan’s investments are stated at fair value, except for its investment contracts, which are valued at contract value which approximates fair value (Note E). Many factors are considered in arriving at that fair market value.  In general, corporate bonds and U.S. government securities are valued based on yields currently available on comparable securities of issuers with similar credit ratings.  Shares of company stock, preferred stock and common stock are valued at quoted market prices.  Registered investment companies are valued at the net asset value as reported by the fund at year end.  Units of common/collective trust funds are valued at the net asset value of the fund, as reported by Vanguard Trust Company, the Trustee of the Plan, on the last business day of the year.   Participant loans are recorded at cost plus accrued interest, which approximates fair value.

Purchases and sales are recorded on a trade-date basis.  Interest income is recorded on the accrual basis.  Dividends are recorded on the ex-dividend date.

In accordance with the policy of stating investments at fair value, the Plan presents in the Statements of Changes in Net Assets Available for Benefits the net appreciation (depreciation) in the fair value of its investments which consists of the realized gains or losses and the unrealized appreciation or depreciation on those investments.

Benefit Payments

Benefits are recorded when paid.

Risks and Uncertainties

The Plan provides various investment options in any combination of stocks, bonds, fixed income securities, registered investment companies, money market funds, and other investment securities.  Investment securities are exposed to various risks such as interest rate, market, and credit risks.  Due to the level of risk associated with certain investment securities, and a level of uncertainty related to changes in the value of investment securities, it is at least reasonably possible that changes in risks in the near term would materially affect participants’ account balances and the amounts reported in the Statements of Net Assets Available for Benefits and the Statements of Changes in Net Assets Available for Benefits.

Reclassifications

Certain reclassifications were made to the prior year financial statements to conform to current year presentation.

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D.            Investments

The following presents the fair value of investments that represent 5 percent or more of the Plan’s net assets.

During 2004 and 2003, the Plan’s investments (including gains and losses on investments bought and sold, as well as held during the year) appreciated/(depreciated) in value as follows:

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E.              Investment Contracts (OneBeacon Insurance Stable Value Fund)

The Plan has entered into benefit-responsive investment contracts with Bank of America, CDC Capital, GE Life and Annuity Insurance, IXIS, John Hancock Mutual, JP Morgan Chase Bank, Massachusetts Mutual, Metropolitan Life Insurance, New York Life Insurance, Rabobank Nederland, State Street Bank, Travelers, and also invests in the Vanguard Prime Money Market Fund. The Stable Value Fund is credited with earnings on the underlying investments and charged for participant withdrawals and administrative expenses. The contracts are included in the financial statements at contract value, which approximates fair value, as reported to the Plan by these institutions. Contract value represents contributions made under the contracts, plus earnings, less participant withdrawals and administrative expenses. The average yields during the years ended December 31, 2004 and 2003 were 4.48 percent and 4.86 percent, respectively.  The crediting interest rates ranged from 2.65 percent to 7.13 percent and 3.55 percent to 7.64 percent at December 31, 2004 and 2003, respectively.  The crediting interest rate on synthetic contracts is based on a formula agreed upon with the issuer, but may not be less than 0 percent. Such interest rates are reviewed on a quarterly basis for resetting.

F.              Related Party Transactions

The Plan invests in shares of mutual funds managed by an affiliate of Vanguard Fiduciary Trust Company (“VFTC”). VFTC acts as trustee for only those investments as defined by the Plan.  The Plan also has investments, which are managed by White Mountains Advisors LLC, an affiliate of the Company. Transactions in such investments qualify as party-in-interest transactions, which are exempt from the prohibited transaction rules. Participants’ loans also constitute party-in-interest transactions.

The Plan invests in the White Mountains Stock Fund (the “Fund”) which is comprised of common shares of White Mountains Insurance Group, Ltd. (the Parent Company) and small amounts of cash invested in the Vanguard Prime Money Market Fund.  The share values of the Fund are recorded and maintained by VFTC, Trustee of the Plan.  During the years ended December 31, 2004 and 2003, the Plan purchased shares in the Fund in the amounts of $5,289,582 and $2,929,945, respectively; sold shares in the Fund of $3,242,290 and $2,019,919, respectively; and had net appreciation in the Fund of $4,894,995 and $2,940,579 respectively. The total value of the Plan’s investment in the Fund was $17,532,825 and $10,596,218 at December 31, 2004 and 2003, respectively.

G.             Tax Status

The Internal Revenue Service has determined and informed the Company by a letter dated January 16, 2002, that the Plan and related trust are designed in accordance with applicable sections of the Internal Revenue Code (“IRC”). The Plan has since been amended. However, management believes the Plan is designed and operates in accordance with the IRC, therefore, no provision for income taxes is required.

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OneBeacon Insurance Savings Plan

Schedule of Assets (Held at End of Year)

(Form 5500, Schedule H, Part IV, Line 4i)

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*                 Denotes party-in-interest.

Cost omitted for participant directed investments.

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Exhibit 99(b)

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We hereby consent to the incorporation by reference in the Registration Statement on Form S-8 (No. 333-68438) of OneBeacon Insurance of our report dated June 28, 2005 relating to the financial statements of the OneBeacon Insurance Savings Plan, which appears in this Form 11-K.

PricewaterhouseCoopers LLP

Boston, Massachusetts

June 28, 2005