In a method of forming an oxide layer, ozone is generated by reacting an oxygen gas having a first flow rate with a nitrogen gas having a second flow rate of more than about 1% of the first flow rate. A reactant including the ozone and nitrogen is provided onto a silicon substrate. A surface of the silicon substrate is oxidized via the reaction of the reactant with silicon in the silicon substrate. The flow rate of the nitrogen gas is increased while ozone serving as an oxidant is formed by reacting the nitrogen gas with the oxygen gas. Thus, the oxide layer or a metal oxide layer including nitrogen may be rapidly formed on the substrate.