Exchange bias effects in CoO/Co bilayers fabricated by ion-assisted deposition were studied as a function of CoO thickness. During the deposition of the top CoO layer, pillar-like CoO structures were embedded in the underlying Co layer due to implantation of oxygen ions. The enhanced coercivity was attributed to the changes in the magnetic reversal mechanism in the ferromagnetic Co layer due to the penetration of pillar-like structures of antiferromagnetic CoO. At low temperature, we found a strong exchange bias field. Our measurements indicate that the exchange bias effect can exist in a nanocomposite system that has a disordered mixture of columnar and planar Co/CoO interfaces. © 2012 The Japan Society of Applied Physics.