Although oregano essential oil has common use in the cosmetics, pharmaceutical, and food sectors, information on its effects on meat quality and shelf life, when incorporated into lamb feed rations, is scarce. In this study, the effects of oregano essential oil (OEO) in feed on the water activity, pH, lipid peroxidation, color parameters, and microbial counts of the Musculus longissimus dorsi (MLD) were investigated. The control group was fed a basal ration and the OEO1 and OEO2 groups were fed a basal ration supplemented with 200 mg/kg and 400 mg/kg OEO, respectively. Akkaraman lambs were fed experimental rations for a period of 56 days and were slaughtered at the end of the fattening period. Lipid peroxidation of the MLD was significantly affected by storage time (ST), group (G) and ST x G, while pH was affected by ST. Lightness was significantly affected by ST, G, and ST x G; redness by ST and G; and yellowness by ST and ST x G. The Micrococcus/Staphylococcus, total mesophilic aerobic bacteria, and total psychrophilic aerobic bacteria counts were significantly affected by ST, G, and ST x G; Pseudomonas spp. counts by ST; and coliform counts by ST and ST x G. The antioxidant and antimicrobial effects of OEO were confirmed.