Surface steps are typically assumed as a source of adatoms for oxygen-chemisorption induced surface reconstruction, but few microscopic observations have been made in the vicinity of steps on reconstructing surfaces. Using in situ scanning tunneling microscopy, we provide direct evidence that surface steps are the source of Cu adatoms for the Cu(1 1 0) (2 x 1)-O restructuring. Using density functional theory, we show that the role of oxygen is to stabilize Cu adatoms detached from step edges via the barrier-less formation of Cu-0 dimers on terraces. Incorporating this atomic process of capturing Cu adatoms into kinetic Monte Carlo simulations reproduces the experimentally observed (2 x 1)-O reconstruction. (C) 2014 Elsevier B.V. All rights reserved.