The evaporative fuel emissions controls on almost every US car since '75 has to have a negative pressure gas tank and fuel return system. The critical components are the gas cap and the charcoal canister check valve, along with having adequate intake manifold vacuum. If there's any leak in these valves (gas tank cap has a check valve in it also) there is more air entering the intake manifold than what's supposed to be there, and the O2 sensor is electronicaly called upon to correct the inproper mixture. When the leak is out of range for the O2 sensor to compensate, the O2 light goes on.
The new US emissions testing requires the gas cap to be tested at every emissions test whereit is placed in a special fixture that finds out how much vacuum it will hold. When it's not screwed on tight, it's like having an intake manifold air leak.
Does that explain?
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon