Addicted2, I absolutely recommend getting the tickets to all the shows. The line for those without tickets can get long, although everyone usually was able to enter. With the ticket, there is no line -- you simply walk in. Print out the schedule, circle a time for each of the shows, and request the tickets. If you choose to deviate, that is not an issue, but if you wish to go, it is just so much easier!
Regarding where to hang out, it depends on what you're looking for. For adults, during the day, the Solarium can't be beat! It's beautiful, quiet, seating areas that are open-air, but shaded or in the sun for tanning, pool, jacuzzis. In the evening, I liked the Rising Tide Bar and the Piano bar. The Globe and Atlas (I think that's what it was called) has live music each evening, and was always a fun place.
We sailed on Oasis of the Seas for 2 weeks in April. Loved it, loved it, loved it!