One of the best Mexican/Oaxacan places in OC is El Montezuma, a hole-in-the-wall in Orange, says robgue, who’s something of a Mexican food snob.
One favorite: pambaso. This sandwich is stuffed with potato, chorizo, and cream, and smothered in chorizo drippings. Mole plate is excellent, and so are enchiladas suizas. Quesadillas (get tinga or rajas) spill over with lettuce, queso fresco, and cream.
They have other Oaxacan specialties, like tlayudas and chapulines (crickets–off the menu). Aguas are all made from scratch.
The joint is pretty plain, but comfortable, not run down. Note that while the name is El Montezuma, the sign out front says La Calle in a holdover from a previous incarnation.
El Montezuma (sign says La Calle) [Inland of OC]
1740 W. Chapman Ave., Orange