HC Is hydrocarbons. It's unburned fuel, so either a misfire (which would be obvious) weak spark, bad compression, bad fuel spray pattern (pressure or clogged injector).
If the O2 was replaced then that shouldn't be the issue. Check the vacuum line to the fuel pressure regulator.
When were the plugs done last?
When was the air filter done last? Can you see light through it?
There's a few things that may help.
91 318is - M50NV (gone)
02 WRB Subaru WRX EJ207 forged, 320 AWHP stock turbo.
85 - 325
02 325i Sedan