It is a native passionflower commonly referred to by some locals as maypops or as purple passionflower. Passiflora incarnata. Native in more northern and eastern parts of the state. I'm not sure how well it does in western OK and southcentral through southwestern OK. I'm in southcentral OK and never have seen it growing in the wild here, but people do plant it in improved soil and keep it happy with irrigation in our hot, dry summers down here.

Thank you, Okiedawn! Very informative! The diversity of Oklahoma's plants never ceases to amaze me.

You're welcome. I agree about OK's native plants. Because our state features so many vastly different soils, there's all kinds of plants native to various parts of the state....some are widespread, some only appear in more limited areas. It amazes me how many of them thrive in what can be some pretty brutal summer weather.

Someone in my old neighborhood has it, be careful, for him it spreads rather well.

Yes, it does that...it should be carefully sited for control or in a place where it doesn't matter that it spreads. It's a native after all, so no harm, and many species benefit from it.