I took my 2003 Toyota Matrix in to the dealership for service. It has 105,000 miles - the oxygen sensor needed replacing, but no other issues found. Now I feel as though I need to shift to a higher gear from 5th gear and that's the highest gear I have. The tachometer is at 3,000 at 70 mph, which seems normal, but I can feel the engine under my foot and I can hear it as well. It also feels somewhat sluggish at that speed, as though the car needs 6th gear. Any ideas?
the oxygen sensor needed replacing
You may have identified the root of all of your issues. Depending on exactly what is happening with your faulty O2 sensor, it could affect any aspect of the intake-combustion-exhaust cycle. I would expect exactly the sort of symptoms that you cite; specifically, rough idle and reduced performance are to be expected.
The oxygen sensor gives feedback to the control unit about how "appropriate" the amount of injected fuel for a given air amount sucked into the cylinders was. If it's faulty, the control unit typically applies some sort of heuristics ("guessing"), so two situations are likely to occur:
less fuel than optimal ("lean" mixture) - this means poor performance
more fuel than optimal ("rich" mixture) - means some fuel left over after combustion and reduced mileage
What's also important, leftover fuel in the exhaust gases can destroy the catalytic converter over time.