The effects of increasing concentrations of oregano (Origanum vulgare L.) and rosemary (Rosmarinus officinalis L.) essentials oil (EO) on ruminal gas emissions were tested in vitro using 50 mL serum bottles. Each bottle contained a 200 mg substrate (alfalfa hay and corn meal 1:1) and a 20 mL solution composed of a buffered medium and rumen fluid (1:2). The percentage of ruminal fermentation products was quantified by an infrared analyzer. The reduction of total gas production was 6% and 9% respectively when using the 1.5 and 2.0 g/L oregano EO measurements. The reduction of methane production was 55%, 72% and 71% respectively with regard to the 1.0, 1.5 and 2.0 g/L oregano EO doses, while rosemary EO (2.0 g/L) reduced the methane production by 9%. The production of ammonia was significantly reduced (59%–78%) by all treatments with the exception of rosemary EO at the lowest dose. Dry matter and neutral detergent fiber degradability was reduced by most of the treatments (respectively 4%–9% and 8%–24%). The total volatile fatty acids (VFA) concentration was markedly decreased by oregano EO and was not affected by rosemary EO. Both EOs mitigated rumen fermentations, but oregano EO gave rise to the highest reduction in methane and ammonia production. However, further research is needed to evaluate the use of these essential oils as dietary supplements by taking into account the negative effects on feed degradability.