I have always done it from below......but on a lift....crawling under there will be insane.......from above? You will have to rotate the engine, but you may also have to remove things to get to it....
And who says it's the O2 sensor? Remember, it can be ANYTHING in the O2 circuit(wire, PCM, sensor, etc.), or, it can be anything that AFFECTS the O2 sensor, like a vacuum leak.....