https://smallbusiness.chron.com/difference-between-public-pr...
https://www.irs.gov/charities-non-profits/eo-operational-req...
"Under the tax law, a section 501(c)(3) organization is presumed to be a private foundation unless it requests, and qualifies for, a ruling or determination as a public charity. Organizations that qualify for public charity status include churches, schools, hospitals, medical research organizations, publicly-supported organizations (i.e., organizations that receive a specified portion of their total support from public sources), and certain supporting organizations."
Edit: Looking at the IRS determination letter from November 3, 2016, OpenAI was organized as a public charity under 170(b)(1)(A)(vi) "Organizations Receiving Substantial Support from a Governmental Unit or from the General Public"
Their last 990 form, filed November 15, 2021, for the calendar year 2020, shows total support over the past 5 years (2016-2020) of $133M, only $41M of which was individual donations of over 2% ($2.6M) so they easily met the 5-year public support test.
Total support includes over $70M in other income in 2018 and 2019 which is missing the required explanation in the 990's. In other words, out of the $92M in public support, $70M is unexplained other income.
Also, Open Philanthropy pledged $30 million in 2017 ($10 per year for 2017-2019) which is considered public support since they are a public charity. However that is more than the $22 in true public support that was reported. Perhaps they didn't complete the pledge.