Oolong tea can best be described as an in-between of green and black tea as it is semi-oxidized. After the leaves are harvested, they are left to dry for a moment in the sun to then be rolled or kneaded into the characteristic shape oolong tea has. This results in oxygen reaching different parts of the leaves to different degrees and they’re oxidized unevenly. Before the leaves are allowed to fully oxidize they are heated to stop the oxidization process, and the oolong tea now has unique traits from both green and black teas.
Easily put, oolong tea is more full-bodied and rounder than green teas but at the same time smoother and lighter than black teas.
Oolong teas can be brewed multiple times and flavors and colors will change after every brew. You can extend the brew time after each time to get a stronger tea and better result. Experiment!