OST is usually a better quality version of what was included in the game. It’s basically what you got if you bought the CDs. Original is what it sounded like on the PS2.
In FFX I think the reorchestrated stuff was overdone and ridiculous but they used a bit more restraint in FFXII and it’s worth checking out. One nice thing is they actually added some unique tracks for places that previously reused tracks from other parts in the game.
Xenoblade Chronicles, X and 2 are also all in world combat.