© 2021 Har Krishan Bhalla & Sons.The present study aimed to investigate the changes in the essential oil contents of some Origanum hybrids grown at different carbon dioxide (400 and 800 ppm) levels in a fully automated carbon dioxide greenhouse. The chemical composition of essential oils was analyzed gas chromatography/ mass spectrometry (GC/MS). It was found that, among the Origanum hybrids, there were differences between the essential oil contents of those grown under natural conditions and controlled greenhouse conditions. While an increase in carbon dioxide level significantly increased the thymol and carvacrol components, which are among important monoterpene components, of Origanum syriacum x O. onites and O. syriacum x O. vulgare hybrids, whereas it led to a decrease in O. syriacum x O. majorona hybrid. To determine the effects on germination of Amaranthus palmeri S. Watson seeds under laboratory conditions, the essential oils were used at 0.5, 1, 2, and 4 μl/petri doses. The most effective doses of essential oils for preventing the germination of A. palmeri seed were found to be 2 and 4 μl/petri. In preventing the germination of seeds, the most effective (100 %) application for all the essential oils was achieved with O. syriacum x O. onites hybrid grown at 800 ppm. These results show that the CO2 increases that may occur as a result of climate change in the next years may lead to changes in the content of essential oils in Origanum hybrids and may have the potential to be used as bio-herbicidal to prevent the germination of A palmeri.