Ozone formation in the gas phase is associated with a large and unusual isotope effect of widespread use in geochemistry and climate research. Little is known whether similar nonstandard mass dependent fractionations also occur in other recombination reactions. Here we report on the pressure and temperature dependence of the isotopic composition of ozone formed by electric discharge in molecular oxygen. Isotope signatures at low pressures show a standard mass dependent depletion, their magnitudes strongly depending on temperature. Our analysis confirms the formation of ozone at Pyrex reactor walls with an atom recombination coefficient gamma = (0.4 +/- 0.1)% at room temperature and slightly higher values at lower temperatures. Thus, although neglected so far, wall assisted ozone formation is an essential part of oxygen plasma chemistry and it could also provide a mechanism explaining the presence of ozone on icy satellites. Recombination reactions on the surface are not likely to show the isotope anomalies associated with ozone formation in the gas phase.