MAF Sensor or 02 sensor?
 

My check engine light came on, so I took it to AutoZone to pull the codes. The guy told me that it's either my MAF sensor or 02 sensor. Are the codes the same so I'll need to do trial and error to see which one it is, or should I write it down and bring it back here?

Write it .

I finally got the print out. It says:
P0420
Definition
Catalyst system efficiency below threshold bank 1
Explanation
Faulty catalystic converter
Probable cause
Rich Air/Fuel ratio
Air leak near rear HO2S
Faulty fuel control HO2S

If the code is all alone, chances are that it is indeed your catalytic converter. Rarely will the O2 cause this code and sometimes it can be caused by a leak in the exhaust or a faulty thermostat. You need a scanner to check your thermostat. It can visually look ok and still not operate as designed. It is supposed to open at 91c.

How does your oil trap hose look, right by the oil filter cap?