What games can xbox one original and series x not play together?

Yes,

Anyone one can be on any of the...

Xbox One,

Xbox One S,

Xbox One X,

Xbox Series S,

Xbox Series X,

and join party's and play the same game together, no matter who is playing on any of these consoles.

I am considering purchasing the new Series X. Before I do I want to know if it is compatible with other Xbox’s when playing Gta 5 Online.

For example, my cousin plays on the Xbox one s and my brother plays on the original Xbox one. Will I be able to play online with them?

Short answer, Yes.

Xbox One,

Xbox One S,

Xbox One X,

Xbox Series S,

Xbox Series X,

They will all be able to play the same games online together, no matter who's on which of these consoles.

I believe what is meant by "full" is that all current gen games and any games that were already made available via the backwards compatibility program (Xbox 360, original Xbox) will be playable on the Series X. So basically "full" means everything you can
currently play on an Xbox One right now you will be able to play on the Series X.

But those 360 and original Xbox games that are not currently backwards compatible still will not be playable on the Series X. That's due to licensing issues with the older games.

You can currently play an original Xbox game, like Halo Combat Evolved or Crimson Skies on the Xbox One, they worked hard to bring our library of favorite games forward and this should continue. The Xbox One and the Series X are by far closer than what
the original Xbox or Xbox 360 was. So the digital copy of The Division 2 I play is going to play on Series X. Plus future games for the next few years are planned to be supported on current systems. Sounds like a great platform for gaming and all the games
we've loved over the years.

Hi, my name is Fernando and I am an independent consultant for Microsoft products.

Cross-play is not supported between GTA 5 Enhanced Xbox Series X|S and Xbox One. So, Xbox Series X|S players using the enhanced version of the game won’t be able to play with those on Xbox One.

By the way , you can play GTA Online with people using systems in the same console family. If you have an Xbox Series X, but your friends own Xbox Ones, you can still jump into lobbies together.