What are the taxes on an IPO?

It's capital gains, which aren't taxed until the capital (IPO stock) is liquidated. I'm sure some people will liquidate their stock promptly but I expect that most will hold it, expecting further growth, and that the big investors, including Altman, will hold onto it for because of the same reason, because they need to sell optimism (what would people think if Altman or Microsoft sold a significant chunk of stock?), and because OpenAI needs lots of assets to build datacenters and buy power.