Exposure of a vicinal Si(100) surface to oxygen at about 10−8Torr for temperatures between about 500 and 700°C produces etching-mediated step recession in competition with oxide island formation. Furthermore, the oxide islands can locally pin the receding steps and thus produce complex surface morphologies. An atomistic lattice-gas model is developed to describe these processes which accounts for the complex interplay between the oxygen surface chemistry and the silicon surface and step dynamics. The oxygen related-processes include dissociative adsorption, diffusion, oxide formation, and etching via SiO desorption. The silicon surface processes include: conversion of single vacancies formed by etching to divacancies and Si adatoms, anisotropic diffusion and aggregation (primarily at step edges) of these divacancies and Si adatoms, and Si ad-dimer attachment-detachment dynamics at steps which reflects anisotropic energetics. Kinetic Monte Carlo simulation of this model allows characterization of the evolving step morphologies. Steps retain some qualitative features of their equilibrium structure, i.e., alternating rough SB steps and smooth SA steps, although etching tends to produce step pairing, and pinning produces protruding “finger” morphologies. These morphological features are seen in scanning tnneling microscopy studies. We also comment on other aspects of evolution such as a mixed pit nucleation and step flow mode, and compare behavior with step flow type growth during Si molecular beam epitaxy.