The effects of lanthanide co-dopants on oxygen diffusion in yttria-stabitized zirconia (YSZ) are studied using a combined first principles density functional theory (DFT)/kinetic Monte Carlo (kMC) modeling approach. DIFT methods are used to calculate barrier energies for oxygen migration in different local cation environments, which are then input into kMC simulations to obtain long-time oxygen diffusivities and activation energies. Simulation results show a substantial increase in the maximum value of the oxygen diffusivity upon co-doping and in the dopant content at which this value is obtained for Lu-co-doped YSZ; while relatively little change is seen for Gd-co-doped YSZ. Examination of the DIFT barrier energies reveals a linear scaling of barrier heights with the size of cations at the diffusion transition state. Using this strong correlation, oxygen diffusivity is examined in YSZ co-doped with several lanthanide elements. The oxygen diffusivity decreases with dopant atomic number (and decreasing dopant ion size) for co-dopants smaller than Y, and changes relatively little when Y is replaced by co-dopants larger than it. These results are broadly consistent with experiment, and are explained in terms of cation-dopant and vacancy concentration-dependent correlation effects, with the aid of a simple analytical model.