Buy Xbox one gamekey and redeem it for Series s?

Can I buy a gamekey for Xbox one via mmoga or cdkeys but redeem it on the Series s?

That's fine

With Xbox, unlike Playstation, it's an overarching system - only the hardware is different. Or all Xbox One games are playable on the Xbox Series X / S.

Can I buy a gamekey for Xbox one via mmoga or cdkeys but redeem it on the Series s?

Yes! As the user @DrachenlordDE already mentioned, the Xbox ecosystem, unlike the Playstation or Nintendo ecosystem, is an overarching system, which only differs from the hardware and the software.

Games that are actually intended for the Xbox One can also be used for the Xbox One S, Xbox One X, Xbox Series S and Xbox Series X.