Antiferromagnetic interlayer coupling is determined from hysteresis loop measurements for Co/Os multilayers with Os thicknesses varying from 4 to 36 angstrom. We found two maxima of antiferromagnetic coupling, which is consistent with an oscillatory behavior with a period of approximately 15 angstrom: this is significantly larger than the period observed for interlayers of Ru, which has a very similar band structure and a topologically equivalent Fermi surface.