From page 31 of Oaxaca Tips
Mesón del Olivo   $$$$   (#2)
Margarita Maza 409, Centro
Daily except Monday: 12 noon - midnight. Cash only
Spanish cuisine is surprisingly rare in Oaxaca, so the Mesón is a welcome presence. The extensive menu offers many options that are appealing and well-prepared, and the typical Spanish décor of wrought iron and bright tablecloths makes for a homey feel.  Jovial owner/chef Javier Olivo (that really is his name!) clearly takes pleasure in his own food, in his dining establishment, and in your enjoyment of both. Try the excellent jamon serrano; Sr. Olivo makes his own . The Mesón is the closest you will get to Spain here in Oaxaca and a satisfying dining choice, especially if you want a change from Mexican fare.