QuickLinks -- Click here to rapidly navigate through this document

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE TO/A

TENDER OFFER STATEMENT UNDER SECTION 14(d)(1) OR 13(e)(1)
OF THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 15)

GOLD RESERVE INC.
(Name of subject company (Issuer))

RUSORO MINING LTD.
(Names of Filing Persons (Offerors))

John H. Riley, Esq.
Gersten Savage LLP
600 Lexington Avenue
9th Floor
New York, NY 10022
(212) 752-9700
(Name, address, and telephone number of person authorized to receive notices and communications on behalf of Filing Persons)

Copies to:

CALCULATION OF FILING FEE

(1)

Estimated solely for the purpose of calculating the registration fee in accordance with Rule 0-11(d) and Rule 0-11(a)(4) of the Securities Exchange Act of 1934, as amended. The transaction valuation is equal to the product of (a) $0.30, which is the average of high and low sale prices of Gold Reserve Class A common shares as reported on the NYSE Alternext US LLC, the principal trading market for Gold Reserve Class A common shares, on December 11, 2008, and (b) the sum of (i) 74,072,185, which is the estimated number of outstanding Gold Reserve Class A common shares as of November 12, 2008 (assuming full conversion or exercise, prior to the Expiry Time of the Offer period, of all outstanding and exercisable options and convertible Gold Reserve Notes for or into Gold Reserve Class A common shares), and (ii) 701,736, which is the estimated number of Gold Reserve Equity Units outstanding as of November 12, 2008, multiplied by (c) the exchange ratio of 3 Rusoro common shares for each Gold Reserve share and Gold Reserve Equity Unit.

(2)

The amount of the filing fee is calculated in accordance with Rule 0-11 of the Securities Exchange Act of 1934, as amended.

ý

Check the box if any part of the fee is offset as provided by Rule 0-11(a)(2) and identify the filing with which the offsetting fee was previously paid. Identify the previous filing by registration statement number, or the form or schedule and the date of its filing.

o

Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer.

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

ý

third party tender offer subject to Rule 14d-1



o

issuer tender offer subject to Rule 13e-4



o

going private transaction subject to Rule 13e-3



o

amendment to Schedule 13D under Rule 13d-2

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

---

        This Amendment No. 15 (this "Amendment") amends and supplements the Tender Offer Statement on Schedule TO filed December 15, 2008 (as amended by the Amendment and previous amendments, the "Schedule TO") by Rusoro Mining Ltd., a corporation existing under the Business Corporations Act (British Columbia) ("Rusoro").

        This Schedule TO relates to the offer (the "Offer") by Rusoro to purchase all of the issued and outstanding Class A common shares (the "Gold Reserve Shares") of Gold Reserve Inc. ("Gold Reserve") and all of the issued and outstanding equity units (the "Gold Reserve Equity Units" and together with the Gold Reserve Shares, the "Gold Reserve Equity"), together with the associated rights (the "SRP Rights") issued under the Shareholder Rights Plan of Gold Reserve, and including any Gold Reserve Equity that may become issued and outstanding after the date of this Offer but prior to the expiry time of the Offer, upon the conversion, exchange or exercise of any securities of Gold Reserve (other than SRP Rights) that are convertible into or exchangeable or exercisable for Gold Reserve Equity. Each Gold Reserve Equity Unit is comprised of one Gold Reserve Class B common share and one common share of Gold Reserve Corporation, a wholly owned subsidiary of Gold Reserve, which Equity Units are convertible into Gold Reserve Shares on a one-for-one basis. Each eligible holder of Gold Reserve Equity will receive 3 Rusoro common shares for each Gold Reserve Share or Gold Reserve Equity Unit validly tendered to the Offer. The Offer is subject to the terms and conditions set forth in Rusoro's Offer to Purchase and Circular dated December 15, 2008, as amended by the Corrected Offer to Purchase and Circular, dated December 15, 2008, filed as Exhibit (a)(1)(R) to the Schedule TO (the "Offer and Circular") and related Letter of Transmittal and Notice of Guaranteed Delivery, copies of which have been filed as Exhibits (a)(1)(B) and (a)(1)(C) respectively, to the Schedule TO, as amended and supplemented by the terms of the Notice of Variation dated January 19, 2009, a copy of which has been filed as Exhibit (a)(1)(S) to the Schedule TO (the "Notice of Variation").

        As permitted by General Instruction F to Schedule TO, the information set forth in the Offer and Circular, the Letter of Transmittal and the Notice of Guaranteed Delivery and the Notice of Variation, including all schedules, exhibits and annexes thereto, is hereby expressly incorporated by reference in response to all items of information required to be included in, or covered by, this Schedule TO and is supplemented by the information specifically provided herein.

        Capitalized terms used herein and not defined herein have the respective meanings assigned to such terms in the Offer and Circular, as supplemented and amended. Except as specifically provided herein, this amendment does not modify any of the information previously reported on the Schedule TO.

Item 12.    Exhibits

        Item 12 of the Schedule TO is amended and supplemented by adding the following:

2

---

SIGNATURES

        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.

Date: January 29, 2009

3

---

Exhibit Index

4

---

---

*

Previously filed.

5

---

QuickLinks

Item 12. Exhibits

SIGNATURES
Exhibit Index