In my opinion there are not any other not to miss on Oahu, but hundreds of nice to do things.
If I only had 1 day and was already going to Pearl Harbor, then I would spend the remainder of the time I had on Waikiki Beach and I would take a sunse sail on the Na Hoku II catamaran that leaves from in front of the Outrigger Waikiki Beach hotel. Then I would find a nice place with a view to eat dinner.
Depends on your interests. I'm active and enjoy outdoors so would do short hike, such as diamond head or makapuu lighthouse, then snorkel/hang out at hanauma bay.