You need to use due caution in Oaxaca. I'm not aware of any bars but you could feel out your hotel desk staff for any recommendations. For sure, stay away from street hustlers. This part of the country is far more conservative than PV, Cabo or Cancun, so think with your big head. If you meet someone, you are far better to get together with them at your hotel than to go off to their place. Good luck, Oaxaca is a great town especially if you see it with a local guide. Enjoy.