Surface oxidation of Cu(100) has been investigated by variable temperature scanning tunneling microscopy and quantitative x-ray photoelectron spectroscopy as a function of O(2) pressure (8.0x10(-7) and 3.7x10(-2) mbar) at 373 K. Three distinct phases in the initial oxidation of Cu(100) have been observed: (1) the formation of the mixed oxygen chemisorption layer consisting of Cu(100)-c(2x2)-O and Cu(100)-(2sqrt[2]xsqrt[2])R45 degrees -O domains, (2) the growth of well-ordered (2sqrt[2]xsqrt[2])R45 degrees-O islands, and (3) the onset of subsurface oxide formation leading to the growth of disordered Cu(2)O. We demonstrate that the (2sqrt[2]xsqrt[2])R45 degrees-O reconstruction is relatively inert in the low pressure regime. The nucleation and growth of well-ordered two-dimensional Cu-O islands between two (2sqrt[2]xsqrt[2])R45 degrees-O domains is revealed by time-resolved scanning tunneling microscopy experiments up to 0.5 ML of oxygen. The formation of these islands and their nanostructure appear to be critical to the onset of further migration of oxygen atoms deeper into copper and subsequent Cu(2)O formation in the high pressure regime. The reactivity of each phase is correlated with the surface morphology and the role of the various island structures in the oxide growth is discussed.