A steel oil ring assembly having an upper rail, a lower rail and an annular space-expander embraced by the rails and having radially extending or axially extending corrugations. The spacer is split at one circumferential location to form abutting end portions which come into pressured contact with each other at a force point located on a plane passing through the centers of the corrugations that extends in an axial direction for radially extending corrugations and in a radial direction for axially extending corrugations. The free ends of the abutting end portions of the spacer form an opening having an angle of 10.degree..+-.9.degree. extending outwardly from the force point.