Oliveria decumbens Vent (Umbelliferae) is a shrub commonly found in the South-East of Iran. Its aerial section is extensively used in herbal medicine. The Disk Diffusion Test and Microbroth Dilution Assay were used to determine the antimicrobial activity of the essential oil from Oliveria decumbens Vent against Staphylococcus aureus. To detect synergy, vancomycin was added to Mueller-Hinton agar at sub-inhibitory concentrations and the inhibitory zones were recorded in millimeters. The main components of oil were thymol (22%), carvacrol (22%) and p-cymene (19%). The O. decumbens oil exhibited strong antistaphylococcal activity (18.0±0.86). Carvacrol was considerably more effective (29.8±1.5) than thymol (17.2±1.13) and p-cymene (0.0±0.0) against Staphylococcus aureus. The oil presented strong synergism with vancomycin (24.9±0.75 vs. 19.3±0.54, p<0.001). However, further studies are required to evaluate its in vivo efficacy.