I went to the OriolesFest last year, and they didn't have a list of guests until you got there. For the current players, they give you about 8 stations to choose from, each with one decent player and two scrubs. The autograph session was only for 2 hours. I picked the line with Brian Roberts (along with Ryan Freel and Matt Albers), so I was only able to get in that one line.
But I think I heard that they're moving it back to the Convention Center this year, so it could be completely different.