It's kind of a weird structure, but I think more like a company than like a nonprofit in the short/medium term. The parent is a nonprofit organization, but it's not the same arrangement as Mozilla Foundation vs. Mozilla Corporation, where all earnings from the subsidiary go to the parent nonprofit. With OpenAI, the parent nonprofit only gets earnings in excess of 100x return. The for-profit company raises investment and returns any earnings to the investors until/unless they hit the 100x return mark. They call this a "capped profit" structure.

When it was founded in 2015, OpenAI was originally a nonprofit, but restructured into this new arrangement earlier this year in order to be able to raise VC money: https://techcrunch.com/2019/03/11/openai-shifts-from-nonprof...