The aim of this study was to evaluate the effects of the level and type of essential oils (EOs) derived from oregano (ORO), thyme (THO), clove (CLO), and cinnamon (CIO), when used as feed additives, on in vitro rumen fermentation and methanogenesis. This was achieved by conducting a metaanalysis that integrated data from relevant studies. A total of 17 articles were included comprising 154 data points. The collected data were subsequently analysed using a mixed model methodology implemented in a SAS software version 9.4. The findings showed that the level of EOs exerted a significant (P < 0.01) linear effect, resulting in increased pH, decreased volume of total gas, CH₄, and NH₃, reduced total volatile fatty acids (VFA) levels, dry matter digestibility (DMD), organic matter digestibility (OMD), and bacterial population. A quadratic effect was also observed with respect to reduced (P < 0.01) CO₂ and propionate (C3) levels. Additionally, the protozoan population demonstrated a linear decrease (P < 0.05), while butyrate (C₄) concentration exhibited a quadratic increase (P < 0.05). However, increasing levels of EOs did not affect the acetate(C₂)- and methanogen-producing bacterial populations. With respect to different EO types, ORO and THO significantly reduced (P < 0.05) total gas (30–41%) and CH₄ production (38–39%) compared to the control. ORO and CLO in turn significantly decreased (P < 0.05) C3 generation (5–7%), while ORO increased C₄ levels (14%) compared to the control. Moreover, CLO, CIO, and THY administration led to a decrease in OMD (8–23%). Due to the limited dataset on microbial population, the diverse types of EOs showed no significant impact on bacterial, protozoan, or methanogen populations. In conclusion, while specific doses of EOs can suppress ruminal methane emissions, they can also inhibit rumen fermentation processes. Among the EOs examined, ORO demonstrated the most potent antimicrobial ability against methanogenesis.