Using the crystal curvature technique we have measured the change in surface stress on Cu(100) induced by oxygen adsorption to produce, at 300 K, a c(2x2) overlayer phase, and at 500 K, the √2x√2)R45^° missing-row reconstructed phase. Density functional theory (DFT) slab calculations have also been performed of the absolute surface stress of the clean Cu(100) surface and these two chemisorbed oxygen phases. Both experiment and theory show that oxygen adsorption leads to a compressive change in the surface stress that is larger for the c(2x2) overlayer (experiment: -1.0 N/m; theory: -3.07 N/m) than for the missing-row reconstruction (experiment: -0.6 N/m; theory: -2.03 N/m). Furthermore, the DFT calculations show that the absolute compressive surface stress of the c(2x2) phase of -1.18 N/m is lowered by the reconstruction to an average value of -0.14 N/m. These results indicate that surface stress reduction plays a role in causing the reconstruction. The discrepancies between theory and experiment are discussed in the context of possible sources of error in both experiment and theory.