I don't have the answer to your question since BF's dad lives there and we don't stay in hotels, but just wanted to say that I've heard great things about "O", and also wanted to highly recommend "Mamma Mia" at the Mandalay Bay (unless you hate ABBA music, in which case you should avoid it!).
I know that for some shows it doesn't matter whether you are a hotel guest or not, but I'm sure each hotel sets it's own policies. If you've spent a lot of money on gambling though, I'm sure they have special seats set aside for those "special" guests!
Have you tried checking online to see what's available? If you go that route, be careful though, there are lots of websites that sell tickets, but charge different service fees. For Mamma Mia, it was most economical to go through Manadalay Bay's website. We booked about a month out and got pretty decent seats, though it would've been nice to be more in the center instead of on the side.