The effects of oxygen pressure on the corrosion of iron embedded in mortar with various Cl− contents were examined by using a novel hyperbaric-oxygen accelerated corrosion test (HOACT). Exposure and electrochemical tests of a pure iron sample embedded in mortar with a cover thickness of 5 mm were performed in HOACT condition with pressurized 100% oxygen gas and with relative humidity of 95%. For comparison, these tests were also performed in ambient air with the same relative humidity. Oxygen reduction current density of iron embedded in mortar increased with an increase in oxygen pressure, suggesting that the corrosion of the embedded iron is expected to be accelerated with oxygen gas pressure assuming an oxygen reduction limited corrosion current density. Iron corrosion in mortar without Cl− was minimal regardless of oxygen gas pressure. In contrast, the corrosion of iron in mortar with Cl− was accelerated with increasing the oxygen gas pressure up to a certain pressure as expected, and the thickness of the rust formed on the iron surface increased with Cl− concentration under each oxygen gas pressure, indicating that Cl− is necessary to initiate corrosion which is enhanced in the pressurized oxygen gas. However, when the oxygen pressure exceeded a certain level, the corrosion was suppressed. Electrochemical impedance spectroscopy measurement using a sensor consisting of a pair of carbon steel electrodes in mortar showed that the charge transfer resistance of the carbon steel under excessively high oxygen gas pressure became high in comparison to that at lower oxygen pressure, indicating the suppression of corrosion initiation is due to formation of a protective passive film under the excessive supply of oxygen. Consequently, the pressurized oxygen gas accelerates the corrosion of iron in mortar with Cl−; however, excess oxygen suppresses corrosion. The optimum condition to efficiently accelerate corrosion of iron in mortar was evaluated.