In a continuing project to characterize natural compounds with activity as modulators of MDR in Staphyloccocus aureus , Osha essential oil and extracts were evaluated. The aim of this work was to identify the active components as MDR modulators in the oil from the roots of Ligusticum porteri Coulter & Rose (Apiaceae). This essential oil was obtained by steam distillation or by solvent extraction and analysed by gas chromatography–mass spectrometry. Forty‐two components were identified. Sabinyl acetate (1) (56.6%), (Z )‐ligustilide (2) (12.9%) and sabinol (3) (3.3%) were the major components of water‐distilled essential oil, while (Z )‐ligustilide (2) (39.1%), sabinyl acetate (1) (34.6%) and 4‐terpinyl acetate (4) (3.1%) were the major components of the dichloromethane extract. At a concentration of 100 µg/ml, the oil from hydrodistillation caused a two‐fold potentiation, and the oil from solvent extraction caused a four‐fold potentiation of the activity of the fluoroquinolone antibiotic norfloxacin against a norfloxacin‐resistant strain possessing the NorA MDR efflux transporter, the major chromosomal drug pump in this pathogen.