The Snakes of North America website from the University of Pittsburgh has photos of dozens of our snakes to help identify the ones you may see in the garden.
This one is most likely a garter snake. They are harmless - they eat frogs, toads, salamanders, earthworms, fish and mice.
Tulsa Master Gardeners website has a discussion of Oklahoma Poisonous Snakes by extension educator, Bruce Peverley.
Every year, when we clean out the flower beds, we see snakes like these. Most likely, they are traveling through our property on their way to a decent hunting ground where rodents are plentiful. This one, I think is a Great Plains Ratsnake. They eat rodents, bats, and birds.
The best site I found is OK Snakes, with photos of all our snakes in categories such as patterned, striped, etc. The one snake I wish we'd attract is one that would eat moles and voles. And, maybe the skunk, raccoon, squirrel, possom or armadillo back there that's making all those holes in the yard for us to trip on.