Material Event (Form 8-K)

As previously disclosed, on December 29, 2025, ON24, Inc., a Delaware corporation (the "Company"), entered into an Agreement and Plan of Merger (the "Merger Agreement") with Cvent Atlanta, LLC, a Delaware limited liability company ("Parent"), and Summit Sub Corp., a Delaware corporation and wholly-owned subsidiary of Parent ("Merger Sub"). The Merger Agreement provides that, upon the terms and subject to the conditions set forth in the Merger Agreement, Merger Sub will merge with and into the Company (the "Merger"), with the Company continuing as the surviving company and a wholly-owned subsidiary of Parent following the Merger. On February 24, 2026, the Company filed a definitive proxy statement (the "Definitive Proxy Statement") with the Securities and Exchange Commission (the "SEC") with respect to the special meeting of the Company's stockholders (the "Special Meeting") scheduled to be held on March 26, 2026, at 8:30 a.m. Pacific Time at the Company's corporate headquarters at 301 Howard Street, Suite 1100, San Francisco, California 94105.

Litigation Related to the Merger

In connection with the Merger Agreement and the transactions contemplated thereby, two putative stockholders of the Company have filed the following lawsuits against the Company and the members of the Company's Board of Directors in the Supreme Court of New York, New York County: (i) Burke v. ON24, Inc., et al., Index No. 651299/2026, filed on March 3, 2026, and (ii) Jones v. ON24, Inc., et al., filed on March 4, 2026. The complaints are substantively similar and variously allege, among other things, that the Definitive Proxy Statement failed to disclose material information. The complaints seek to enjoin the Company from proceeding with or consummating the proposed Merger and seek to recover damages in the event the proposed Merger is consummated. In addition, between February 11, 2026 and March 19, 2026, a series of demand letters and certain associated draft complaints were also sent to the Company by counsel for purported stockholders of the Company, alleging deficiencies in the Company's disclosures.

The Company believes that the allegations in the complaints and letters described above are without merit. The Company also believes that the disclosures set forth in the Definitive Proxy Statement comply fully with all applicable law, and do not need to be supplemented. Nevertheless, solely to avoid the nuisance, risks, costs, and uncertainties inherent in disputes concerning these types of allegations, including the possibility that any such claim could delay or adversely affect the Merger, and allow the Company's stockholders to vote on the Merger at the Special Meeting, the Company has determined voluntarily to supplement certain disclosures in the Definitive Proxy Statement with the supplemental disclosures set forth below. Nothing in this Current Report on Form 8-Kshall be deemed an admission of the legal merit of the various litigation matters described above or the allegations made therein, or of the necessity, or materiality under applicable laws, of any of the disclosures set forth herein.

Supplemental Disclosures

The following supplemental disclosures should be read in conjunction with the Definitive Proxy Statement, which should be read in its entirety. The inclusion in the supplemental disclosures of certain information should not be regarded as an indication that any of the Company or its affiliates, officers, directors or other representatives, or any other recipient of this information, considered, or now considers, it to be material, and such information should not be relied upon as such. Defined terms used but not defined herein have the meanings set forth in the Definitive Proxy Statement. For clarity, new text within restated paragraphs from the Definitive Proxy Statement is highlighted with bold, underlined text, and deleted text within restated paragraphs from the Definitive Proxy Statement is highlighted with strikethrough text.

The disclosure under the heading "The Merger-Background of the Merger" is hereby supplemented by adding to the end of the first full paragraph on page 34 of the Definitive Proxy Statement the following sentence:

The non-disclosureagreements also did not contain any "don't-ask don't-waive" or similar provisions which would apply after public announcement of the execution of an acquisition agreement by ON24 with a third party such that no counterparty to any such non-disclosureagreement would be limited from making an offer or entering into discussions, negotiations, agreements or understandings with respect to a possible transaction resulting from an acquisition inquiry or Acquisition Proposal and/or Superior Proposal.

The disclosure under the heading "The Merger-Background of the Merger" is hereby supplemented by adding to the end of the second paragraph on page 36 of the Definitive Proxy Statement the following sentence:

Cvent's proposal did not include any provisions regarding the participation or employment of the Company's management in the surviving corporation, nor was it conditioned on any such participation or employment, nor were there any negotiations related to any such participation or employment prior to the signing of the Merger Agreement.

The disclosure under the heading "The Merger-Background of the Merger" is hereby supplemented by adding to the end of the last paragraph on page 38 of the Definitive Proxy Statement the following sentence:

No negotiations related to the participation or employment of the Company's management in the surviving corporation occurred prior to the signing of the Merger Agreement and Cvent's request that certain key members of ON24's executive management team provide transition support was not implemented in connection with finalization of the Merger Agreement.

The disclosure under the heading captioned "The Merger-Opinion of ON 24's Financial Advisor" is hereby amended and supplemented by adding the following to the second full paragraph on page 48 of the Definitive Proxy Statement (with new text in bold and underlined):

Goldman Sachs derived ranges of illustrative enterprise values for ON24 by adding the ranges of present values it derived above. Goldman Sachs then added the amount of ON24's cash and cash equivalents of approximately $169.0 million as of September 30, 2025 (after adjusting for cash used for share repurchase through December 26, 2025), as provided by and approved for Goldman Sachs' use by the management of ON24, to derive a range of illustrative equity values for ON24. Goldman Sachs then divided the range of illustrative equity values it derived by the number of fully diluted outstanding shares of ON24 common stock of approximately 49.5 million as of December 26, 2025, as provided by and approved for Goldman Sachs' use by the management of ON24, using the treasury stock method, to derive a range of illustrative present values per share of ON24 common stock, rounded to the nearest $0.10, ranging from $6.30 to $8.30.

The disclosure under the heading captioned "The Merger-Opinion of ON 24's Financial Advisor" is hereby amended and supplemented by adding the following to the first full paragraph on page 49 of the Definitive Proxy Statement (with new text in bold and underlined):

Goldman Sachs then added the amount of ON24's cash and cash equivalents as of December 31,2026 and 2027 of approximately $168 million and $189 million, respectively, each as provided by and approved for Goldman Sachs' use by the management of ON24, to the respective implied enterprise values in order to derive a range of illustrative equity values as of December 31 for ON24 for each of the fiscal years 2026 and 2027. Goldman Sachs then divided these implied equity values by the projected year-end numberof fully diluted outstanding shares of ON24 common stock as of December 31, 2026 and 2027 of approximately 52.196 million and 55.225 million, respectively, calculated using information provided by and approved for Goldman Sachs' use by the management of ON24, to derive a range of implied future values per share of ON24 common stock (excluding dividends). Goldman Sachs then discounted these implied future equity values per share of ON24 common stock to September 30, 2025, using an illustrative discount rate of 12.0%, reflecting an estimate of ON24's cost of equity. Goldman Sachs derived such discount rate by application of the CAPM, which requires certain company-specific inputs, including a beta for ON24, as well as certain financial metrics for the United States financial markets generally. This analysis resulted in a range of implied present values per share of ON24 common stock, rounded to the nearest $0.10, of $3.90 to $7.60.

The disclosure under the heading captioned "The Merger-Opinion of ON 24's Financial Advisor" is hereby amended and supplemented by restating in its entirety the table set forth on pages 49 and 50 of the Definitive Proxy Statement (with new text in bold and underlined):

The disclosure under the heading captioned "The Merger-Opinion of ON 24's Financial Advisor" is hereby amended and supplemented by adding the following to the first full paragraph on page 50 of the Definitive Proxy Statement (with new text in bold and underlined):

The foregoing analysis indicated a range of EV/NTM Revenue multiples ranging from 0.9x to 4.0x, and a median EV/NTM Revenue multiple of 2.3x. Based on the results of the foregoing calculations and Goldman Sachs' professional judgment and experience, Goldman Sachs applied a reference range of EV/NTM Revenue multiples of 1.0x to 2.3x to ON24's estimated NTM Revenue as of December 31, 2025, as provided by and approved for Goldman Sachs' use by the management of ON24, to derive a range of implied enterprise values for ON24. Goldman Sachs then added the cash and cash equivalents of ON24 of approximately $169.0 million as of September 30, 2025 (after adjusting for cash used for share repurchase through December 26, 2025), as provided by and approved for Goldman Sachs' use by the management of ON24, and divided the result by the number of fully diluted outstanding shares of ON24 common stock of approximately 49.5 million as of December 26, 2025, as provided by and approved for Goldman Sachs' use by the management of ON24, to derive a reference range of implied values per share of ON24 common stock, rounded to the nearest $0.10, of $6.20 to $9.70.

The disclosure under the heading captioned "The Merger-Opinion of ON 24's Financial Advisor" is hereby amended and supplemented by restating in its entirety the table set forth on page 51 of the Definitive Proxy Statement (with new text in bold and underlined):

The disclosure under the heading "The Merger-Certain Financial Projections" is hereby supplemented by amending and restating the table on page 57 of the Definitive Proxy Statement as follows:

The Projections are set forth in the following table:

Additional Information About the Acquisition and Where to Find It

This communication is being made in respect of the proposed Merger involving ON24, Parent and Merger Sub. ON24 filed a definitive proxy statement with the SEC. ON24 commenced mailing of the definitive proxy statement to its stockholders entitled to vote at the Special Meeting relating to the Merger on or about February 24, 2026. ON24 may also file other relevant documents with the SEC regarding the Merger and related matters. This communication is not a substitute for the definitive proxy statement or any other document that ON24 may file with the SEC. INVESTORS AND STOCKHOLDERS OF ON24 ARE URGED TO READ THE DEFINITIVE PROXY STATEMENT AND ANY OTHER RELEVANT DOCUMENTS THAT MAY BE FILED WITH THE SEC BY ON24, AS WELL AS ANY AMENDMENTS OR SUPPLEMENTS TO THESE DOCUMENTS, CAREFULLY AND IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT ON24 AND THE MERGER. Investors and stockholders will be able to obtain a free copy of the proxy statement and other documents containing important information about ON24 and the Merger once such documents are filed by ON24 with the SEC at the SEC's website at www.sec.gov or from ON24 at its website at https://investors.on24.com/overview/default.aspx.

Participants in the Solicitation

ON24 and certain of its directors and executive officers may be deemed to be participants in the solicitation of proxies in connection with the Merger. Information about ON24's directors and executive officers and their direct and indirect interests, by security holdings or otherwise, is set forth in the definitive proxy statement for the Merger referenced above, under the headings "Executive Compensation," "Employee Benefit and Equity Compensation Plans" and "Equity Compensation Plan Information." Additional information about the directors and executive officers of ON24, their ownership of ON24 common stock, and ON24's transactions with related persons is set forth in (i) ON24's Annual Report on Form 10-K for thefiscal year ended December 31, 2024, including under the headings "Item 10. Directors, Executive Officers and Corporate Governance", "Item 11. Executive Compensation", "Item 12. Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters" and "Item 13. Certain Relationships and Related Transactions, and Director Independence", which was filed with the SEC on March 13, 2025, and can be found here, (ii) ON24's definitive proxy statement for its 2025 annual meeting of stockholders, which was filed with the SEC on April 29, 2025, under the headings "Director Compensation", "Executive Officers", "Employee Benefit and Equity Compensation Plans", "Executive Compensation", "Security Ownership of Certain Beneficial Owners and Management", and "Relationships and Related Party Transactions" and can be found here, and (iii) ON24's subsequently filed Current Reports on Form 8-Kand Quarterly Reports on Form 10-Q,as well as ON24's Annual Report on Form 10-Kfor the fiscal year ended December 31, 2025 filed on March 12, 2026. To the extent holdings of ON24 securities by its directors or executive officers have changed since the amounts set forth in the disclosures identified above, such changes have been or will be reflected on Forms 3, 4 and 5, filed with the SEC (which are available here). Other information regarding the participants in the solicitation may be set forth in other relevant materials to be filed with the SEC. Investors and stockholders should read the definitive proxy statement and such other

relevant materials carefully before making any voting or investment decisions. Copies of the documents filed with the SEC by ON24 will be available free of charge through the website maintained by the SEC at sec.gov and ON24's website at https://investors.on24.com/overview/default.aspx.