We have investigated how the conductance of Si(100)-(2 x 1) and Si(111)-(7 x 7) surfaces change during exposure to molecular oxygen. A monotonic decrease in conductance is seen as the (100) surfaces oxidizes. In contract to a prior study, we propose that this change is caused by a decrease in surface mobility due to surface roughening. The oxidation of the (111) surfaces induces a more complex conductance variation. This can be explained by changes in the band bending which occur during the reaction. Our results show a two-stage reaction, the first stage being completed after one monolayer of oxygen has been absorbed.