If OpenAI remains a 501(c)(3) charity, then any employee of Microsoft on the board will have a fiduciary duty to advance the mission of the charity, rather than the business needs of Microsoft. There are obvious conflicts of interest here. I don't expect the IRS to be a fan of this arrangement.
Others have pointed out several reasons this isn't actually a problem (and that the premise itself is incorrect since "OpenAI" is not a charity), but one thing not mentioned: even if the MS-appointed board member is a MS employee, yes they will have a fiduciary duty to the organizations under the purview of the board, but unless they are also a board member of Microsoft (extraordinarily unlikely) they have no such fi…
OpenAI is a 501c3 charity nonprofit, and the OpenAI board under discussion is the board of that charity nonprofit.
OpenAI Global LLC is a for-profit subsidiary of a for-profit subsidiary of OpenAI, both of which are controlled, by their foundational agreements that gie them legal existence, by a different (AFAICT not for-profit but not legally a nonprofit) LLC subsidiary of OpenAI (OpenAI GP LLC.)