Amendment: SEC Form SC TO-I/A filed by White Mountains Insurance Group Ltd.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

AMENDMENT NO. 1

TO

SCHEDULE TO

Tender Offer Statement under Section 14(d)(1) or 13(e)(1)

of the Securities Exchange Act of 1934

WHITE MOUNTAINS INSURANCE GROUP, LTD.

(Name of Subject Company (Issuer) and Filing Person (Issuer and Offeror))

Common Shares, par value $1.00 per share

(Title of Class of Securities)

G9618E107

(CUSIP Number of Class of Securities)

Robert L. Seelig, Esq.

Executive Vice President and General Counsel

White Mountains Insurance Group, Ltd.

23 South Main Street, Suite 3B

Hanover, New Hampshire 03755-2053

Telephone: (603) 640-2200

(Name, address and telephone number of person authorized to receive notices and communications on behalf of filing persons)

With a copy to:

Check the appropriate boxes below to designate any transactions to which the statement relates:

Check the following box if the filing is a final amendment reporting the results of the tender offer: ¨

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

INTRODUCTION

This Amendment No. 1 (this “Amendment”) amends and supplements the Issuer Tender Offer Statement on Schedule TO, initially filed by White Mountains Insurance Group, Ltd., a company organized under the laws of Bermuda (the “Company”), with the Securities and Exchange Commission on November 21, 2025 (as amended as of the date hereof, the “Schedule TO”), which relates to the offer by the Company to purchase up to $300 million in value of its issued and outstanding Common Shares, par value $1.00 per share (the “Common Shares” or the “Shares”), at a purchase price not greater than $2,050 nor less than $1,850 per Share, in cash, less any applicable withholding taxes and without interest, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated November 21, 2025 (the “Offer to Purchase”), and in the related Letter of Transmittal.

This Amendment is being filed in satisfaction of the reporting requirements of Rule 13e-4(c)(4) promulgated under the Securities Exchange Act of 1934, as amended. Except as otherwise set forth below, the information set forth in the Schedule TO, including all exhibits thereto that were previously filed with the Schedule TO, remains unchanged and is incorporated by reference as relevant to the items in this Amendment.

Item 11. Additional Information.

(1) Item 11 of the Schedule TO is hereby amended and supplemented by adding the following to the end thereof:

On December 22, 2025, the Company issued a press release announcing the preliminary results of the Offer, which expired at 12:00 midnight, New York City time, at the end of the day on December 19, 2025. A copy of the press release is attached hereto as Exhibit (a)(5)(O) and incorporated herein by reference.

Item 12. Exhibits.

(1) Item 12 of the Schedule TO is hereby amended and supplemented by adding the following exhibit:

(a)(5)(O) Press Release, dated December 22, 2025

SIGNATURE

After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

December 22, 2025

INDEX OF EXHIBITS

*            Previously filed as exhibits to the Schedule TO