This study evaluated the application of low levels of ozone at medium consistency to improve the properties of old corrugated container (OCC) pulp. Pulps obtained from kraft liner and corrugating medium were also tried. Ozone reduces the kappa number of the pulp, but the surface charge of the fiber is not changed. Ozonation clearly increases the relative bonded area (RBA) in sheets made from treated fibers. It was shown that RBA is essential for Concora medium test (CMT) resistance. Papermaking properties like tensile strength, internal bond strength, compression strength, and CMT were significantly increased even at the lowest ozone charge employed. For OCC pulp, it is shown that similar improvements can be reached by refining and alkali treatment. Nevertheless, alkali has a greater effect on pulp freeness and produces a higher charge in the effluents.