Oolong teas are teas that are semi-oxidized. This could mean some teas are oxidized at 18% and look like green teas or they could be oxidized at 80% and look like black teas.
The production process is similar to black teas. Once the leaves have withered, and they're malleable, it's time to start oxidizing the teas. The leaves are bruised and slowly heated. The bruising and heating both slowly encourages oxidation and stops it.