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Cover - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Mar. 04, 2022
Jun. 30, 2021
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2021    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-39965    
Entity Registrant Name ON24, INC.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 94-3292599    
Entity Address, Address Line One 50 Beale Street, 8th Floor    
Entity Address, City or Town San Francisco    
Entity Address, State or Province CA    
Entity Address, Postal Zip Code 94105    
City Area Code 415    
Local Phone Number 369-8000    
Title of 12(b) Security Common Stock, par value $0.0001 per share    
Trading Symbol ONTF    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company true    
Entity Ex Transition Period false    
ICFR Auditor Attestation Flag false    
Entity Shell Company false    
Entity Public Float     $ 735
Entity Common Stock, Shares Outstanding   47,777,947  
Documents Incorporated by Reference Portions of the registrant’s definitive Proxy Statement for the 2022 Annual Stockholders’ Meeting, which the registrant expects to file with the Securities and Exchange Commission within 120 days of December 31, 2021, are incorporated by reference into Part III (Items 10, 11,12, 13 and 14) of this Annual Report on Form 10-K.    
Amendment Flag false    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Entity Central Index Key 0001110611