My 2002 outback back fires and hesitates some times when starting out.
The only code that has showed up is oxygen senser. The problem goes away sometimes for day's

Sounds odd, you might want to post this question over at the site in my sig.

2002, get it into the dealers for warranty!!! Unless you have mega miles on the clocks or warrany has expired.

replace the front o2 sensor (air/fuel ratio sensor)
and have the ecu reflashed to the latest version.if the reflash isnt done,or doesnt have the latest reflash the new sensor will fail soon.also check the plug wires for arcing and pull each plug wire of checking for corrosion on the coil terminal.