Adsorption and reaction of N2O on a stepped Ni(755) surface have been investigated by temperature-programmed desorption (TPD). The decomposition of adsorbed N2O occurs exclusively at the step sites on Ni(755) below 200 K during TPD ramp, yielding gaseous Nz and leaving atomic oxygen there. The amount of the atomic oxygen is controlled easily by changing the exposure of N2O. The character of the atomic oxygen has been studied by using decomposition of cycloheptane as a probe reaction. All the atomic oxygen generated in situ from coadsorbed N2O can react with carbon atoms produced from the decomposition of cycloheptane, resulting in desorption of CO around 600 K upon heating. However, two thirds of the predeposited atomic oxygen cannot be desorbed as CO upon heating up to 723 K. The predeposited atomic oxygen becomes inactive during the predeposition process and probably interacts with the step sites more strongly.