So, after some reading, I think I may have this straight.
As an exhaust valve opens, exhaust gas is pushed out toward the catalytic convertor, creating positive pressure that pushes gas out the tailpipe. At this point, there is a momentum of air that is flowing out the exhaust. When this exhaust valve closes, the air flowing in the tailpipe will continue to flow based on its momentum and a siphon effect is created. This siphon effect creates a very short duration vacuum very close to the exhaust head. The pressure is lowest at the exhaust valve and increases toward the catalytic converter.
So, if a leak exists at the exhaust head gasket, then fresh air could be pulled into the exhaust at that point because there is a vacuum, and thus affecting the O2 sensor reading. But, the further away from the valve and the head gasket one goes, the less likely that this vacuum will have any effect on drawing in fresh air.
So, my assumption is that exhaust leaks at the exhaust manifold gasket or very close to the exhaust manifold gasket could influence O2 readings, but cracks further along the exhaust course (say, at the flange) would not influence O2 readings.