A simplified model was derived for magnetizing force convection of oxygen gas and solved for transient characteristics of oxygen gas in a vertical open pipe. Oxygen gas in the cylindrical pipe initially flows out downward since oxygen gas is heavier than air. However, the magnetizing force works to attract the oxygen gas in the lower half of the vertical pipe, and the oxygen gas rises back to the central part of the pipe where the magnetic field is strongest. After a long time, all the oxygen gas in the pipe is replaced with air due to diffusion. This model represented moderately well the transient concentration of oxygen gas measured experimentally in a similar system.