This is a linkpost for http://effective-altruism.com/ea/1g9/should_we_be_spending_no_less_on_alternate_foods/
I tried looking in the IRS Form 990 dataset on Amazon S3, specifically searching the text files for forms published in 2017 and 2016.
I found no match for (case-insensitive) openai (other than one organization that was clearly different, its name had openair in it). Searching (case-insensitive) "open ai" gave matches that all had "open air" or "open aid" in them. So, it seems like either they have a really weird legal name or their Form 990 has not yet been released. Googling didn't reveal any articles of incorporation or legal name.
Their 2016 Form 990 just surfaced: http://www.guidestar.org/FinDocuments/2016/810/861/2016-810861541-0eb61629-9.pdf says they had $13m income in 2016, and at the end, assets of ~$2.6m.