Mechanical milling has been used to synthesize ferromagnetic (FM, Co) - antiferromagnetic (AFM, NiO) composites. The coercivity, HC, and energy product, BHMax, of these composites can be enhanced at room temperature after appropriate heat treatments above the Néel temperature of the AFM, TN. Although the maximum Hc is achieved for the (NiO)1: 1 (Co) weight ratio, BHMax is further enhanced for the (NiO)2:3(Co) ratio, where higher saturation magnetization is obtained due to the larger amount of FM. Exchange coupling, responsible for these effects, decreases as the temperature is increased and vanishes close to TN. The thermal stability of the coercivity enhancement remains rather insensitive to the somewhat broad distribution of blocking temperatures of this system.