Electronic structure calculations have been employed to study the associative desorption of preadsorbed atomic oxygen from a calcium oxide (100) surface. Two reaction mechanism were considered, corresponding to symmetric and asymmetric reaction paths. The energy barrier of the latter was found to be the lower of the two. The barrier for the asymmetric reaction was found to be sensitive both to the location of the nearest calcium ions and to the distance between the surface oxygen ions, and is argued to be strongly affected by surface temperature. The desorption mechanism is shown to be conceptually similar to the previously addressed N2O decomposition and CO2 formation reactions. Catalytical aspects are stressed as oxygen desorption from CaO(100) is compared to those of nickel and platinum.