Abstract The chemical composition of the oils from two endemic and rare taxa of the genus Origanum L., i.e O. calcaratum Juss. = O. tournefortii Aiton and O. scabrum Boiss. et Heldr., from Greece, were investigated by GC and GC/MS. Twenty-two compounds were identified into the oil of O. calcaratum, representing 98.0% of the oil composition. The main compounds were p-cymene (18.1%), Î³-terpinene (9.6%), thymol (42.8%), carvacrol (12.9%) and isocaryophyllene (4.7%). In the oil of O. scabrum, 28 compounds were identified, representing 98.0% of the oil composition. The main compound was carvacrol (66.7%), while p-cymene (7.8%), Î³-terpinene (3.6%), caryophyllene oxide (2.1%) and manool (2.7%) were found in large amounts. The oils were investigated for their antimicrobial activity against Gram-positive, i.e., Staphylococcus aureus, S. epidermidis and Gram-negative, i.e., Escherichia coli, Klebsiella pneumoniae microorganisms.