There are towels at the main pool at OPR. No pool house/locker room but, there are bathrooms to change in at the main pool, inside OPR.
Also, after you check-in, you can take your room towels to your 'island' pool and, mousekeeping will replenish your towels. Beach towels, from what I've read, are good if you're going to the water park(s) (to mark your chairs) and, if you're going there and bringing your towels, roll the towels, pack them in a jumbo ziploc and SQUEEZE the air out! It makes 'em smaller.
I am so jealous!!! Have a great time!