I made a thread about this long ago when it was first revealed that Tobi was Obito.
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that's it if you're interested.
Anyway basically the evidence I used in the thread for how Obito survived was that he used Kamui because of what Madara said here
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It's possible he awakened his MS at that moment because of physical need. It's possible for a Uchiha to awaken a Sharingan in a moment of great stress/need and be unable to use it again afterwards. Evidence of this is here, where Sasuke awakens a 1 tamoe sharingan the night of the uchiha massacre
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After this, Sasuke was unable to use the sharingan again, until it once again awakened (this time with 2 tamoe) during the Zabuza/Haku fight.
As far as I'm concerned this is pretty solid evidence that is how Obito survived the boulder. He probably would have died had Madara not saved his life with Zetsu cells, but the Kamui was the reason why he was even still alive at all when Madara found him.
There is also the possibility that he is able to use his S/T jutsu with a 3 tamoe. While there is evidence of this I don't believe it to be true. The main reason why he was shown using his S/T with just a 3 tamoe was so that we couldn't see his Mangekyo sharingan. Seeing the mangekyo would be a dead give-away for who Tobi is since it is the same as Kakashi's. Kishi wanted to keep it a mystery at that time.