| Cover | Dec. 01, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Dec. 1, 2021 |
| Entity Registrant Name | ON24, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-39965 |
| Entity Tax Identification Number | 94-3292599 |
| Entity Address, Address Line One | 50 Beale Street, |
| Entity Address, Address Line Two | 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $0.0001 per share |
| Trading Symbol | ONTF |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001110611 |
| Amendment Flag | false |
