I've lived in a few different regions in the US, and found that OKC has really good food comparatively.
Pizza
Hall's Pizza Kitchen
Empire Slice House
Upper Crust
The Wedge
many others
Tacos - lots of "street taco" places around here, and also Central American food, not just Tex-Mex.
Taco Nation
1 million others if you can find them. Go to the places where they don't speak English, and its cash only.
honestly, I'm still searching for the best Mexican / Tex-Mex in this area. There are infinite options.
Fish - seafood can be more sparse around here, but not impossible to find. Most of the sit-down restaurants have salmon and other fish on the menu. Charleston's, Cheevers, etc....