It isn't explicitly stated anywhere in the book (that I recall).
However, in the book, Parzival has to do a lot of traveling around within the OASIS to get from place to place. The implication is that players go back to where they left the last session. At the start of the book, Parzival appears in school on Ludus, where he's stuck because he can't afford a ship or the teleportation fees to go anyplace else.
Note that in the book, there are chatrooms (Aech's one in particular) where they can jump to, but they still continue their OASIS session when they exit the chatroom.
I'd really recommend that you read the novel.