The safety analysis of High-Temperature and Very High Temperature gas-cooled Reactors requires reliable estimates of nuclear graphite gasification as a function of temperature, among other parameters, in the unlikely event of an air ingress accident. Although the rates of the prevailing chemical reactions increase exponentially with temperature, graphite gasification at high temperatures is limited by the oxygen diffusion through the boundary layer. The effective diffusion velocity depends on the total flow rate and pressure of the bulk air-gas mixture. This paper develops a semi-empirical Sherwood number correlation for calculating the oxygen diffusion velocity. The correlation is based on a compiled database of the results of convective heat transfer experiments with wires and cylinders of different diameters in air, water and paraffin oil at 0.006 ≤ Re ≤ 1,604 and 0.068 ≤ Sc ≤ 35.2, and of mass transfer experiments at 4.8 ≤ Re ≤ 77 and 1,300 ≤ Sc ≤ 2,000. The developed correlation is within ± 8% of the compiled database of 567 data points and consistent with reported gasification rate measurements at higher temperatures in experiments using different size specimens of nuclear graphite grades of NBG-18 and NB-25, IG-11, IG-110 and IG-430 in atmospheric air at 0.08 ≤ Re ≤ 30. Unlike the Graetz solution that gives a constant Sh of 3.66 at Re ≤ 1.0, the present correlation shows Sh decreases monotonically to much lower values with decreasing Re.