Intuition would tell you that the sun always sets in the west, but on the island of Oahu, Hawaii, the sun sets on the North Shore. In reality the area referred to as the "North Shore" is located more toward the western part of the island.
The best place to watch the sunset is appropriately called Sunset Beach, and if the swells are up you can watch the sunset behind the surfers trying to catch the last waves of the day.
In my opinion Sunset Beach is also a great place to stay if you are vacationing in Hawaii. There are a lot of houses for rent in this area and I prefer the vibe of the North Shore to that of Waikiki and Honolulu.