You would hear a boost leak if it was at the connector where it clamps on the tmic to TB.
IMO a bad O2 will not cause a misfire.
If it was to rich, car should run like crap.
You may need to pull the spark plugs, how long have they been in there ? they should be replaced every 60,000 miles.
199,000 May 12 new ej257 short block, Aug 11 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. CHECK your oil, these cars use it.
Engine Build - Click Here