First off, my experience in Oklahoma is that IF you can get vegetarian food at the generic chains is in generally not very good bordering on terrible. The best example of this are the "Mexican" restaurants around town. If you eat at a large chain there is generally lard in the beans and chicken broth in the rice (rice is sketchy is everywhere), but locals almost exclusively use not animal oils--no lard, no bacon.
El Rio Verde (Mexican), which is actually quite authentic as far as Mexican goes, or Cafe Ole (New Mexican) which is fairly authentic New Mexican cuisine (I lived in Santa Fe), but they could add a little salt to just about everything they make--add the salt, you will be glad you did.
Elote Cafe specializes in local foods but I have never been that impressed; they are worth a try, but only after the previous have been attempted.
Really just try local Mexican restaurants around town, they are pretty generic but most are somewhat accommodating.
You will not be disappointed at those in Blue:
Ri Le or Binh Le (Vietnamese)- the owner of Ri Le is vegan, will make the Nuoc Cham (sauce) vegetarian for you, and will express giddy delight when he figures out you are vegetarian. I have taught many people to crave Vietnamese with the Deluxe Soy Bun w/ vegan sauce.
Be Le, as mentioned above, is interesting for the reason that they have all kinds of crazy fake meats, but the previous two taste better. I love the owners of Be Le along with Ri Le.
Lanna Thai (Thai)- will make any dish vegetarian to vegan, just tell them how vegetarian to go. They can actually be trusted.
Keo - more expensive and modern than those above, also not as delicious. It is also a little more difficult here but they are willing to adapt any dish but it might take some talking --which would seem odd as they are the only Caucasian staffed restaurant (I say this having had plenty of experience with Vietnamese and Thai restaurants both here and in SE Asia).
Japanese food here is pretty generic, you can get what you would normally expect to get at pretty much any of them.
India Palace and Kolam are the Indian restaurants in town. The former is the old tradition and may be a little better, but you can always support the little guy. They are the general Indian places--vegetarian is easy to get and the food is good.
Pure Cafe (raw vegan) -the only raw vegan restaurant I know of in Oklahoma. They might exist elsewhere, and aren't cheap, but if you have the cash you should try it out. Just don't go looking for a "normal" meal. The food is very rich.
Cosmo Cafe is good on Brookside and they have yummy vegetarian items on the menu - I go for the deserts though, try the Guinness Brownie. They also have drinks.
Garlic Rose is a supposed of staple here, but for the price, the quality of food is quite lame. You will have just as good of luck at Olive Garden--and that is not a compliment. I have yet to try the sister restaurants.
I would like to try the Italian restaurant mentioned in previous posts. There is the local Napoli, but it too, is just generically bad Italian. Non-pizza Italian in Oklahoma does not impress me, in ANY way (so far).
Hideaway, Umberto's, Pie Hole in that order.
The biggest suggestion I can give: don't eat at non-local chains--they are terrible.
My girlfriend is telling me to say that I cook the best food in town, so maybe you should just come to my house.
I'm really interested in any restaurant I haven't listed here. If it is good and it isn't here then I probably haven't been there, so please let us all know. For denser locations of decent food check out Cherry Street & Brookside. Utica Square isn't that great-considering price and lack of extensive vegetarian menu options (w/ the exception of the Queenie's grilled cheese). Things like the Riverwalk should just be avoided for their crowds and complete generic-ness.