Exposure of Rh(111) to atomic oxygen leads to the facile formation of a full-coverage and ordered (1X1)-O monolayer which is stable at room temperature. This result differs markedly from the half-coverage (2X1)-O overlayer which forms at saturation when using molecular oxygen. This demonstrates that kinetic rather than thermodynamic constraints inhibit the formation of dense oxygen overlayers when O-2 is the oxidant. We also report that O absorption into the bulk proceeds much more readily when using O rather than O-2, a finding with direct implications for enhanced methods of low-temperature metallic oxidation. These results demonstrate that there are important fundamental differences in the way in which low-energy beams of atomic and molecular oxygen interact with metals.