Oxygen availability governs the cathodic reactions during the corrosion propagation stage. However, existing studies often neglect the coupled effects of heat, moisture, chloride, and oxygen transport, leading to inaccurate predictions of corrosion rates. This study developed a multi-field coupled model that integrates heat-moisture transfer, chloride transport, and oxygen diffusion-consumption dynamics to investigate the role of oxygen availability in steel corrosion. Accelerated corrosion tests were conducted under cyclic temperature conditions of 20−40 °C and humidity levels of 40%−80% to monitor key parameters, including oxygen concentration, chloride ions distribution, and corrosion current density. Results reveal that due to cathodic consumption, the oxygen concentration at the steel surface is significantly lower than that in the surrounding concrete. High temperature and humidity reduce oxygen availability by limiting diffusion and solubility, while chloride ions accelerate depassivation and indirectly influence oxygen consumption. A sharp decline in oxygen concentration, coupled with a rise in corrosion current density, was observed before and after steel depassivation.