Summer in Oklahoma can be brutal, especially if you own a two story home. There are several causes for an over heated second floor. First, heat rises. As your HVAC pushes cool air into your downstairs, this cool air pushes the hot air upstairs. This causes the upstairs unit to have to work even harder to cool the second story. Your roof may also be contributing to the hot second floor. If your roof is exposed to direct sunlight the heat is absorbed and passed down through the attic and into your second floor. You could also have poorly installed, leaking or old duct work that prevents the cool air from being delivered to your upstairs rooms.
Tulsa heat and air company recommends the following:
Keep filters changed and cleaned regularly
Check window caulking
Circulate the air upstairs with fans