Wildfire spread models that couple physical transport and chemical kinetics sometimes simplify or neglect gas-phase pyrolysis product oxidation chemistry. However, empirical evidence suggests that oxygen (O2) is available for gas-phase and solid-phase combustion within the flaming reaction zone. This study addresses outstanding questions of O2 availability by directly measuring O2 concentrations near fuel surfaces within spreading fuel bed fires for the first time. Temporally and spatially resolved O2 concentrations within laboratory fires were investigated using uniform fuel beds of medium density fiberboard (MDF) and cardboard (CB) combs at various packing ratios (β) and angles. Minimum O2 concentration reached approximately 2-5 mol% for most fuel beds of higher β (> 0.010), while some fuel beds of low β (≤ 0.010) exhibited higher minimum O2 concentrations up to 8 mol%. Flame rate of spread, residence time, and minimum O2 were found to vary with β and were influenced by bed angle within a given β. Thermogravimetric analysis was used to assess O2 availability impacts on flaming and smoldering fuel consumption rates. Negligible impacts were observed for initially unburned fuels at ambient O2 concentrations of ≤ 10 mol%, but O2 availability had significant impacts on char oxidation rates at all tested concentrations. A one-dimensional diffusion model of O2 availability at char surfaces for near-particle O2 concentrations tested and measured in TGA and fuel bed experiments revealed a strong dependence of O2 availability on mass flux from the particle. The results of this work provide new insights regarding O2 availability and inert assumptions for fire spread models.