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O2 could be read with a multimeter on the signal wire, or even better with an oscilloscope to graph the pattern.
On that note.. even more helpful, get the short/long term fuel trim numbers. Find someone with a scanner and get the datastream pulled up.
Chances are you'll see o2 be OK.. probably, but fuel trim will be VERY high. That would indicate clogged injectors or carboned intake/valves. If o2 is reading lean and fuel trim is also reading rich you could have an exhaust leak before the o2 or weak spark. If you have ok o2, but rich fuel trim at idle, rev it up... if fuel trim goes normal you have a vacuum leak.
Start with those if you'd like.
Poor gas mileage could come from many things that won't mess up emissions. Unless the extra gas that's being added is coming out the tail pipe it won't fail. You could have a carboned up intake/valves absorbing all the injecter fuel, requiring it to add more, for example. And remember the cat eats up loooots of HC that would otherwise get let out and fail your emissions if it's running rich (causing poor gas mileage). Too many variables for an exact answer, but it's certainly possible.
Really I can't answer that question again, without the fuel trim numbers / o2 readings. Those are absolutely invaluable for that.
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