Water extraction from soybean during soymilk and tofu production produce a by-product known as okara. Protein in okara can be considered as source of bioactive peptides that have antioxidant. However, this parent proteins have to be hydrolyzed using protease from microorganisms in solid state fermentation. R. oligosporus is a mold that usually used for making tempeh and is known to have proteolytic activity. The aim of this research was to find the optimum condition of okara fermented with R. oligosporus in different fermentation times (24, 48, 72, 96, and 120 hours), R. oligosporus initial concentrations (1.04×10-3, 1.95×10-3, 2.44×10-3 gDCW/gokara), and water activity values (0.913 and 0.931). The optimum condition was selected based on highest antioxidant activity. The results showed that the highest antioxidant activity was 46.28% at 5000 ppm with degree of hydrolysis was 17.80% with 5 days fermentation, initial inoculum concentration of 1.04×10-3 gDCW/gokara, and water activity (aw) of 0.931. The increasing of antioxidant activity was caused by the increasing of cysteine, as it was the amino acid that known to have antioxidant activity. Essential amino acids such as lysine, methionine, and tryptophan were also increased after undergone solid state fermentation using R. oligosporus.