There are towels provided at the pools at OKW. They are small-ish towels, just large enough to wrap around an average-sized person's hips but you can take as many as you need.
For the water parks, you can rent towels there for $2 each. They are the same towels as the ones at the resort pools. They sell large beach towels in the shops at the water parks if you want a big towel.
I wouldn't take the OKW room towels to the water parks for a number of reasons. First, if you are staying at OKW on points you get fresh towels only every four days. I wouldn't want to have to reuse those sandy towels in the room after taking them to a water park. Also, if you forgot to bring them back from the water park or lost them or they were taken by someone else, the OKW owners would have to pay to replace them.