How do I get from Mexico DF to Oaxaca? Is Interjet the only no frills service that flies that route?
I wouldn't say Interjet is a "no frills" airline as it has free food and drink with very comfortable new planes. I would say it is on a par with an airline like BA and way ahead of any U.S. airline. Their prices start from 190 USD return.
Vivaaerobus used to fly to Oaxaca but have stopped (they were very cheap but very unreliable). Aeromexico fly that route but are way more expensive (but do give you a lot more options). Many people go by first-class bus - 6 hours for 35 USD each way.
I'd second the idea of the bus but would also suggest you consider the Ejecutivo bus as it is a bit more expensive than first class but a lot more comfortable. You can probably leave around midnight, sleep all the way and arrive about 6AM. Saves a night in a hotel as well.