We were just at POR last week and my kids swam every night when we got back from the parks. I know it was well after 10:00 most nights, but call and ask to be sure!
I know when we stayed at the Saratoga we would come back from the parks around 11:00p and there would be families in the pools, both quiet as well as the big ones. I even remember the slides were still running.
tom
POR Ol Man Island the lifeguards there are on duty 10am to 8pm I believe. Slide is only open during this time. Pool I believe is 7am-10pm. Quiet pools yeah pretty much open all the time!