I bought 2 inch thick oak decks that have been seasoned for a year outside. I would like to start gluing them together i.e. to make a bench top (and eventually a whole woodworking bench)
What do I risk? Is it possible that if I glue it together it will later crack or deform?
Your assumptions are correct: the wood will move as it acclimatizes to any indoor space, and your glued-up joints and panels might move a lot as it dries. Depending on total moisture content now and in your shop (and, of course, exactly how the joints are made, since wood moves more across the grain than with it) it might not be a disaster, but if that lumber decides to move, there isn't going to be any glue joint in the world that keeps it from moving.
Even if it was covered outside, the moisture content will be a lot higher than most indoor spaces in North America. Though, since you are making a bench, it is possible that you are in a partially heated shop, which might be a bit more damp. Still, the moisture content would ideally be in the single digits (<10%) before you start working it, and for installation in a fully heated indoor space closer to 6%.
To reduce checking and cracking, the simplest thing you can do is to stack and sticker it indoors for about a month prior to working with it. You should paint the ends with end-grain sealant so you slow down drying that can cause checking. (The usual advice here is to not use latex paint, but proper end-grain sealant.)
