2010-06-15 : If the targeted creature is a token, it will cease to exist after it’s put into its owner’s library.

2010-06-15 : If the targeted creature is an illegal target by the time Oust resolves, the spell doesn’t resolve. No one gains life.

2010-06-15 : If the targeted creature’s owner has no cards left in their library, that creature is put into that library as the only card there.

2010-06-15 : If the targeted creature’s owner has one or more cards in their library, that creature is put into that library directly under the top card.

2010-06-15 : Note that the creature’s controller, not its owner, is the one who gains life.