The objective of this in vitro study was to assess the effects of high-carvacrol oregano oil (ORE; 50 g/kg oregano oil, 800–802 g/kg carvacrol), on microbial fermentation and CH₄ production. In the experiment (a complete randomized block design), treatments included a negative control (CTL, no additive), a positive control (monensin, MON, 10 mg/L of culture fluid), and ORE (20, 40, 80, 120, 200, 160, and 1000 mg/L of culture fluid). Compared with CTL, MON shifted (p < 0.05) volatile fatty acid (VFA) profile from acetate and butyrate to propionate production, thereby reducing (p < 0.05) CH4 production (−26%). Monensin decreased (p < 0.05) NH3 and branched-chain VFA concentrations. Of the doses evaluated, only the highest dose (1000 mg/L) affected ruminal microbial fermentation and CH4 production. At this dose, ORE reduced (p < 0.05) gas production, total VFA, acetate, propionate and NH₃ concentrations, and CH₄ production (−22%). The reduced gas production and total VFA is an indication of feed digestion inhibition. These results suggest that ORE may decrease CH₄ production and improve ruminal N utilization. However, these findings need to be validated in vivo to determine the optimal dose to benefit from the positive effects while avoiding the negative impact of ORE on feed digestion.