Ruminant livestock contributes significantly to global methane (CH4) production and its mitigation is of great importance. Feed additives represent a cost-effective and realistic means of achieving this. Previous research demonstrated that slight elevations of the rumen oxidation reduction potential (ORP) using oxygen-releasing feed additives serves to hinder enteric methanogenesis. This is due to the niche specialisation of ruminant methanogens, which are typically only active at ORPs below -300 millivolts. In-vitro assessment of these compounds, including both calcium and magnesium peroxide (CaO2, MgO2), and encapsulated liquid H2O2 for controlled, slow release has demonstrated effective CH4 mitigation potential, with consistent CH4 reductions of >50% observed. Encapsulated formats of CaO2 and MgO2 offer potential feasibility as a CH4 mitigation feed additive solution in both intensive and pasture-based production systems. However, the impact these oxygen-releasing compounds have on the rumen microbiome, specifically what might be occurring when hydrogen (H2) or carbon dioxide (CO2) are diverted away from ruminant CH4 production requires further investigation. This study focuses on the influence of ORP modulating compounds on in-vitro rumen microbial communities. Nucleic acids were co-extracted from rumen fluid. Amplicon sequencing of the 16S and 18S genes was performed on all samples (n=64) comprising inoculum (time zero), CaO2, MgO2, liquid H2O2, encapsulated liquid H2O2, and controls, over the course of a 21-day RUSITEC trial. Microbial community dynamics were integrated with process data and revealed the extent to which ORP can alter the rumen microbial community, thus elucidating the microbial mechanisms which underpin the CH4 reductions observed.