Thanks for all the replies. The horses are pretty poor and wild. I don't think they're worth much and may have been purposely abandoned. We spoke with the one neighbor that has horses
The closest thing to a pertinent law I have found says it's up to the sheriff to sell the horses and make the landowner whole if the horses are in the sheriff's possession. I haven't found anything about what the law is if a landowner has unclaimed horses on their property. I'll call the Oklahoma Cattle Association and possibly an attorney Monday if the sheriff still refuses to get involved. I may or may not post a found notice in the local paper for two weeks.
I'll be sure to update this thread for future reference if I find any definite answers.