Oxygen supply is a critical parameter in 3D cell cultivation using bioreactors. Since bioreactor designs often prioritise practical constraints, understanding the oxygen supply dynamics of the media is crucial for achieving either uniform or spatially controlled oxygen delivery. This is particularly relevant for cultivating chondrocytes in hypoxic conditions. To evaluate oxygen supply, we used a concentric bioreactor with a central flow channel (140 mm length). Phosphate-buffered saline (PBS) served as the medium, which was deoxygenated with nitrogen to create an oxygen sink. Oxygen diffusion in an 1.5% agarose gel bed was experimentally measured using fluorescence quenching oxygen detectors and compared to finite element simulation results. Additionally, two cavities in the gel were filled with collagen to mimic a cell-free osteochondral niche. Measurements showed a gradual oxygen decrease with distance from the flow channel, reaching concentrations below 0.025 mM after 150 h. Reoxygenation occurred faster, with levels exceeding 0.15 mM within 50 and 80 h in setups without and with collagen, respectively. Simulation results initially matched experimental data only during initial or late stage of de-/reoxygenation, but were improved by varying the diffusion coefficient and the mass transfer coefficient for liquid-to-gel transitions. Further refinements could include batch-specific diffusion coefficients and concentration-dependent diffusion adjustments. Understanding oxygen dynamics in bioreactors will enable precise oxygen delivery, particularly when integrating proliferating stem cells or chondrocytes as oxygen sinks. These findings pave the way for more effective bioreactor designs tailored to specific cell culture needs.