In this research the properties, the phytotoxicity and the antifungal activity of four Greek oregano essential oils were discussed. In a field trail the oregano essential oil production was remarkable higher at the first year at the second harvest (spring harvest) compared to the first harvest (autumn). Compared to the origin less total phenols composition were observed from the oregano biotypes originated from the mountain Olympus compared to the oregano derived from Central Greece and Ios island At the applied doses of 97, 256 and 610 mu l oregano essential oils per plant, less plant development and yield were observed whereas the applied dose of 16 mu l caused no damage and promote tomato plant yield. In vitro all oregano biotypes proved a strong antifungal effect against important tomato plant pathogens. The essential oil from the mountain Olympus, "Leptokaria" biotype, was proved the most effective.