Subaru Repair: o2 sensor/ recent service, wheel hub, gas mileage

Question
I brought my 2000 outback to a new subaru guy (that I haven’t used before) for the 90k service, to diagnose a check engine light that had been intermittently coming on and off over the previous few days, and for a squealing sound emanating from the front passenger side wheel when turning left. The mechanic performed the service, diagnosed that I needed a new front O2 sensor, and replaced the wheel hub. I told him to hold off on replacing the O2 sensor because he had to order one and I couldn’t leave the car. Since the trip to the shop, I have been experiencing a few problems: when I first start driving the car I can hear a knocking or tapping sound from the engine that goes away in a few minutes, my gas mileage has dropped from about 18/22 to 15/18, the car idles rough and shimmies while stopped and in gear (it stops if I put it into neutral), occasionally all power will cut out during acceleration for a few seconds and then return, and finally, the car just feels “sluggish”. However, all this was going on without the check engine light. For several weeks the check engine light didn’t come back until recently, so I called up the shop and have an appointment to bring it in to replace the sensor. Should replacing the O2 sensor remedy all of these or are there other more sinister things that are going on and need to be checked out?  

Answer
I would have the O2 sensor replaced, you always need to fix the known problems first.  I would say it will fix you gas mileage, that is what the O2 sensor does, it reads if the car is running rich or lean.  If its reading wrong it could be giving it more fuel than needed, thus using more fuel.  Hope the O2 sensor helps!