Hmmm...try going to the home page on the link I sent you and checking through all the snakes that occur in Oklahoma. It may not be a Natricine.
And yes, as a general rule, some snakes can change color and pattern quite a bit as they age. Keep that in mind. If you see a look alike on the page I sent you, do an image search for the species on google.com to see a bunch more pics.
Let me know what you find. I'm really interested in knowing now.