I was wondering once I have finished missions like Oasis and In To The Pitt does anything change in time, like does the wasteland become more like oasis and is the pitt cured of illness ? im just wondering,

In story yes. In actual game appearances. Nothing save acknowledgements of you completing the quest.

In Oasis, if you take the increase growth path, the Leaf Mother says that 'the Wasteland will become green in decades rather than centuries.' But either way you won't see any 'decades' pass in game-time, unless your really obsessive and wait 3000 times.

For the Pitt, if you go back, they're always still doing research. Presumably this will take years as well.