Depends on what part of the state.
Southeast is lumber and forest products. Northwest is hay and wheat farming. Southwest cattle ranching.
Even though it's been producing for over 100 years, we still have a fair amount of oil & gas production across the State. Because of being centrally located, it is the pipeline hub of the U.S.
Because the wind never stops blowing (See reason), we have a lot of wind farms in the Western side of the state.
The wind never stops blowing in Oklahoma because Texas sucks, and Kansas blows...
Probably our biggest industry is Indian Casinos!
We have the World's largest casino in Thackerville, OK. It warms my heart to see the parking lot full of cars with Texas license plates. We appreciate you wealthy Texans donating money to our state coffers!