For example, if I tried to trade an Oshawott from a W2 game to a SS game, would it work? Because it's not in the National Dex.
No it wouldn't work for one thing SS is 4th gen while Oshawott is 5th gen so its doesn't exist yet. If you have played 5th gen before you know about the transfer where they come but don't return
The answer is – drumroll please–…
It will not work because the coding for Oshawott is not in the coding for SoulSilver, because Oshawatt was designed after SS was created. Even Game Freak can't see the future.