I baked 2 9x11 cakes in glass pans. I coated them with veg. oil and flour. Now they seem very stuck. How can I get them out without breaking them?

if they are completly cooled, try warming them up a bit in the oven at a low temp. This should help loosen cakes a bit. You can also try running a knife around the sides (be careful not to cut the cake). Hope this helps. I have become a believer in Baker's Joy. It's quick, covers evenly, and haven't had a cake stick yet. (Knock on wood.)

The cakes have been out for about 20 mins

Sounds like they sat in the pans too long. I never leave mine for more than 10 mins before turning them out. Try to warm them up a bit and see if this helps.