Layered oxides (AMO2, where A = Li or Na and M = transition metal) are essential positive electrode materials for lithium- and sodium-ion batteries. A fundamental question in ion transport is whether Li+ or Na+ diffuses faster in these materials; however, distinguishing intrinsic diffusion properties from the effects of particle size and electrode composition is challenging. Using operando muon spin spectroscopy and molecular dynamics simulations, we determined the Li+ and Na+ self-diffusion coefficients in O3-LixCoO2, O3-NaxCoO2, and P2-NaxCoO2. Our findings revealed that Na+ diffusion is higher in the P2-type structure than in the O3-type structure primarily due to weaker electrostatic interactions. In the O3-type structure, Li+ diffuses faster than Na+, whose larger ionic size hinders mobility. These insights clarify the ion transport mechanisms and advance the design of next-generation battery materials.