EMT pay in okc stinks (8.00/hr or so) but if you're looking for experience EMSA is the way to go.
1) Call to the hospitals and see if they hire EMT's to work in the ER's. They call the position an "ER Tech". ParkView Hospital in El Reno operates their EMS out of the hospital. When not on a call, they work in the ER. Good experience and not too far away.
2) Security Guard agencies. Sometimes they will hire an EMT to work as a guard. Easy job. Kinda boring, though.
3) Transportation services. Some of the wheelchair transportation companies hire EMTs. Not a bad job. Just drive a van around and pick up people who are wheelchair-bound, roll them into to van, drop them off at their location, head off to the next client. It's an interesting job. Lots of neat people with interesting stories. MedRide and Trinity are two places that pop into mind.
4) Call the smaller towns surrounding the OKC area to see if they're hiring EMT's. Might have to drive a bit but most of those places have 24 hour shifts but the plus is you won't have to drive as frequently as a mon-fri job. I worked at Prague, my 1st real EMT job.
If all else fails, you can call the paramedic programs and see if they've heard about some openings somewhere.