Fugitive and residual methane emissions from landfills are driven by subsurface gas transport pathways that challenge effective in situ mitigation. Methane oxidation biosystems (MOBs) provide a managed mitigation approach by promoting microbially mediated oxidation within porous, organic-rich cover materials; however, their performance is governed by gas transport, moisture conditions, and methanotrophic activity. This study evaluates compost overs, a coarse residual waste material generated during industrial compost screening and compares its performance with finished compost as oxidation-layer substrates for landfill MOBs. Laboratory investigations integrated batch incubations, column experiments under varying methane loading, and physicochemical characterization of microbial activity and gas transport. An ongoing pilot-scale study using an engineered sloped biocover integrates surface flux measurements, subsurface gas profiling, and continuous environmental monitoring to assess mitigation performance under field-relevant forcing. Results show complete methane removal for both materials at moderate loading (118 g m-2 d-1), whereas under elevated loading (236 g m-2 d-1) compost overs sustained higher oxidation efficiencies despite lower intrinsic methanotrophic activity, reflecting transport-controlled microbial oxidation supported by higher air-filled porosity, effective gas diffusivity and improved oxygen availability. Inert nitrogen profiles provided a diagnostic indicator of oxidation zone behavior, distinguishing diffusion from advection-dominated transport. Pilot-scale measurements demonstrate near-complete removal during low loading (50 g m-2 d-1), with spatial–temporal variability linked to moisture redistribution, gas transport, and climatic drivers. Overall, the results demonstrate that transport properties regulate methane oxidation dynamics and highlight the potential of compost overs as a viable waste-derived substrate for landfill methane mitigation.