Spending two months a couple miles from the Mexican border this winter pretty much spoiled us for authentic Mexican food.
No hablo engli. Look at a pic on the menu and point.
There's some decent places around to find it in OKC, but it ain't places like San Marcos, Chelinos, or Abuelos. More along the lines of what Cohiba mentioned.
I know a few around Tulsa as well. El Refugio Azteca, Calaveras, Tacos El Rinconcito, Tacos Don Francisco, Tacos San Pedro, Mr Tacos, El Rio Verde (popular but not my pick over the others listed). Casa San Marcos has lots of good seafood items.
Casa Tequila is a bit fancier, more like a typical restaurant. And maybe a bit more Tex-Mex ish in nature, but they still have some traditional fare and a much larger drink menu if you or the wife wants a big margarita or something. We go there since it's around a miles away and the wife and kids are more comfortable there.