• Mechanisms of bulk and local oxygen transport in cathode catalyst layers (CCLs) of PEMFCs are systematically presented.
• State-of-the-art strategies to mitigate oxygen transport resistance in CCLs are reviewed, including novel structure design, carbon supports design, and ionomer design.
• New directions for oxygen transport development in anode catalyst layers (ACLs) of PEMWEs are inspired by PEMFCs.
• The review highlights the critical role of pore structure and catalyst–ionomer agglomerates in oxygen transport for both PEMFCs and PEMWEs.