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[mjezek_imp]

At the risk of sounding stupid, is there a difference between the O2 sensor and the PAIR valve?

Nope it is not, .... if you search for it, you will actually find a lot of info about it here in this forum... but here is the short version:
The PAIR Valve is required ( mostly by california law ) to keep emission legal, it is a mechanism/valve adding fresh air to the exhausted air. It's mounted on the cylinder head.
O2 sensor is actually installed in the exhaust header just before the catalytic converter. ECU uses the reading of this sensor to create a air/fuel mixture perfectly suited for optimum function of the catalytic converter which is far away from the optimum fuel/air ratio for best performance.
That's why when you buy a PCV you get a bypass plug for the 02 sensor, otherwise the PCV and the ECU would constantly "fight" over the air/fuel ratio reading ...