I apologize if this is your welcome experience to Oaxaca. The rest of your visit will probably be much more enjoyable. I do have a report from one visitor who heard that travelers attempting to walk to the gate are in danger of robbery. I find this surprising since the area is rather public and subject to frequent traffic during aircraft arrivals. This sounds like a story invented by the airport taxi service.
Car rentals are available at the airport.
I suggest calling the international number to arrange your rental in advance. My car rental experiences here have been pleasant. See Transportation and car rental tips for more information. Control Tower Torre de Control 958-19006.