The in situ Mg-sialon in low-carbon MgO-C refractories was studied with respect to its oxidation behavior and mechanism at 1500 °C. The results indicated that the oxidation index and rate constant of low-carbon MgO-C refractories with Mg-sialon were 26.2% and 0.51 × 10⁻³ cm²/min at 1500 °C for 2 h, respectively. The formation of a dense MgO-Mg₂SiO₄-MgAl₂O₄ protective layer contributed to considerable oxidation resistance, and the generation of this thicker layer was due to the combined volume effect of Mg₂SiO₄ and MgAl₂O₄. The reduced porosity and more complex pore structure were also found in the refractories with Mg-sialon. Therefore, further oxidation was restricted as the oxygen diffusion path was effectively blocked. This work proves the potential application of Mg-sialon in improving the oxidation resistance of low-carbon MgO-C refractories.