Each side rail of a combined oil ring, which includes a spacer-expander and two side rails, has an ion nitriding treatment layer on an outer peripheral surface of the rail. The inner peripheral surface of each rail may either be a bare surface or have either a soft surface treatment or a hard chrome plating. The upper and lower surfaces of the rail may either be a bare surface or have a soft surface treatment. The method comprises coiling a strip to form a coil, placing the coil on a jig so that only the outer peripheral surface is exposed to the ion nitriding and then cutting the coil to form the individual rails with a gap.