Quoting galant1517:
Wohoo! Good question! This depends on how bad the compression was.
Extreme case:
The one cylinder has compression that is so bad that it isn't firing anymore. You (hypothetically, not actually), being the dumbass that you are, are still driving this thing. Oh, and you are still feeding that cylinder with fuel, because the injector is still plugged in. None of this fuel is being burned, which means the majority of it is being pushed into the exhaust. The O2 sensor sees this, and thinks that the car is running rich, and the ecu responds by leaning everthing out.
So the actual situation is that it goes rich out of the tail pipe, until the computer corrects for it, and then the remaining cylinders go lean.
If the fuel is not burned completely or only partially burned, then the air is also not burned completely or only partially burned which will increase the O2 content in the exhaust stream. It will flag the O2 sensor to read lean and cause the ecu to give more fuel. O2 sensor doesn't sense fuel, it senses O2 (air).
Same scenario why a missfire always reads lean.