The objectives of this work were to investigate the effects of unconventional oils rich in phenolic compounds and rumen adaption on methane (CH4) gas production and rumen fermentation characteristics under in vitro rumen conditions. For this purpose, two sets of trials were conducted. In the first trial, the effects of blackberry, blueberry, raspberry, pomegranate, black seed and hemp oils on CH4 production and fermentation were examined in three 24 h batch culture experiments. Treatments in each experiment consisted of control (no oil supplement), control plus corn oil, or control plus two of the unconventional oils. Oils were added to rumen cultures at 500 mg/L (equivalent to 3.3 g oil/kg of diet dry matter (DM)). After 24 h of incubation, CH4 production was not different between the control and the corn oil treatments. Of the six unconventional oils tested, only hemp and blueberry oils reduced (P<0.05) CH4 production by 9-16% relative to the control and corn oil treatments. No significant differences were observed between treatments in dry matter digestibility (DMD) or total volatile fatty acids (tVFA). Except for a reduction (P<0.05) in acetate concentration with the raspberry oil, and an increase (P<0.05) in valerate concentration with the pomegranate oil, all other treatments had similar VFA concentrations. In the second trial, the effects of adding oregano essential oil (OEO) to adapted and unadapted rumen cultures on CH4 production and rumen fermentation were evaluated under in vitro condition. Rumen cultures were obtained from continues culture fermenters fed a control diet or control diet plus OEO at 250 mg/day for 10 days. The addition of OEO decreased (P<0.05) ii CH4 production only in adapted cultures. Total VFA and acetate concentrations were greater (P<0.05) in the unadapted than adapted cultures and their concentrations decreased (P<0.05) with the addition of OEO particularly when added to the adapted cultures. Propionate concentrations were also greater (P<0.05) in the unadapted than the adapted cultures and concentrations decreased (P<0.05) with the addition of OEO. Dry matter degradability and total gas production decreased (P<0.03) with the addition of OEO in both cultures and total gas production tended (P<0.13) to be lower when added to the adapted cultures. In conclusion, our results showed that hemp and blueberry oils were moderately effective in reducing rumen CH4 formation without compromising rumen fermentation and digestibility. Oregano Essential oil addition negatively affected rumen fermentation in both adapted and unadapted cultures and the effect was greater in the adapted cultures. The greater effects of OEO on CH4 production in the adapted cultures most likely due to the lower fermentation efficiency in these cultures.