The oxidation of deuterium-, acetylene-, and ethylene-terminated Si(100) by gas-phase atomic oxygen is evaluated using Auger electron spectroscopy (AES). The atomic oxygen is generated by the 157 nm laser photolysis of O-2, which results in nominally a 50/50 mix of the ground (P-3) and electronically excited (D-1) state species. O atom adsorption probabilities for oxygen coverages in the 1-3 ML regime are estimated to be on the order of 0.1 for these adlayer systems. For both C2H2 and C2H4 adlayers no effective loss of surface carbon is observed, indicating that there first must be extensive oxidation of the near-surface region of silicon (greater than about 3 ML) prior to the removal of chemisorbed carbon on Si(100). The O/Si AES ratio for the 0 atom oxidation for oxygen coverages of about 1-3 NIL in the near-surface regime is found to be linear with 0 atom exposure for all three adlayers, with the propensity for reaction being given by the relative slopes of 1.0:1.4:2.0 for D-, C2H2-, and C2H4-terminated Si(100), respectively. The relatively low rate for O atom oxidation of D-terminated Si(100) suggests that the presence of defects facilitates the oxidation. While the difference in the oxidation rates for the C2H2 and C2H4 adlayers is statistically significant, this is most likely not due to initial adlayer coverage effects the specific cause(s) of this difference, however, is not established.