What's the consideration for this contract?
Consideration is almost meaningless as an obstacle here. They can give the other party a peppercorn, and that would be enough to count as consideration.
https://en.wikipedia.org/wiki/Peppercorn_(law)
There might be other legal challenges here, but 'consideration' is unlikely to be one of them. Unless OpenAI has idiots for lawyers.
Employment Contract the First:
We are paying you (WAGE) for your labor. In addition you also will be paid (OPTIONS) that, after a vesting period, will pay you a lot of money. If you terminate this employment your options are null and void unless you sign Employment Contract the Second.
Employment Contract the Second:
You agree to shut the fuck up about everything you saw at OpenAI until the end of time and we agree to pay out your options.
Both of these have consideration and as far as I'm aware there's nothing in contract law that requires contracts to be completely self-contained and immutable. If two parties agree to change the deal, then the deal can change. The problem is that OpenAI's agreements are specifically designed to put one counterparty at a disadvantage so that they have to sign the second agreement later.
There is an escape valve in contract law for "nobody would sign this" kinds of clauses, but I'm not sure how you'd use it. The legal term of art that you would allege is that the second contract is "unconscionable". But the standard of what counts as unconscionable in contract law is extremely high, because otherwise people would wriggle out of contracts the moment that what seemed like favorable terms turned unfavorable. Contract law doesn't care if the deal is fair (that's the FTC's job), it cares about whether or not the deal was agreed to.