In presented study total phenolic contents, antioxidant capacities and their bioaccessibilities from cookies supplemented with oleaster flour were investigated. Oleaster flours (OFs) were produced using two different methods (peeled oleaster flour: POF and unpeeled oleaster flour: UPOF) from two different genotypes. OFs were used to replace wheat flour in the cookie formulation (control) at the levels of 5, 10, 15, 20 and 25% (w/w). According to the results, enrichment of OFs clearly increased total phenolic contents, antioxidant capacities and bioaccessibilities of cookies. The highest bioaccessible antioxidant capacities (ABTS, CUPRAC, and FRAP) of the samples were obtained from cookie samples enriched with 25% UPOF-1. In conclusion, the increases in phenolic contents, antioxidant capacities, and bioaccessibilities from cookies supplemented with OFs suggest the potential enhancement of beneficial health effect of cookie due to increased content of bioactive compounds present in oleaster flour.