There’s a gym called Health Zone, which is a part of the Saint Francis Health System. They have a basketball court, but I’m not sure about leagues. They had decent pickup games, though. I haven’t played there since high school.
There used to be Champions Sports Complex, a long time ago. I think they held AAU games there. That has since closed, so that doesn’t help you the slightest bit.
You can also Google search, as well. Most YMCAs will have good pickup games. The one is Bixby is actually decent.
I would try maybe looking for some close to Midtown/Downtown. I say this because there is a higher concentration of younger people around those neighborhoods, and so you might find some good ones around there.