The complete vaporization of a three-dimensional submicron liquid oxygen (LOX) droplet into quiescent environments comprised of either hydrogen or helium has been simulated using molecular dynamics. The environment pressures and temperatures ranged from 2 to 20 MPa and 200 to 300 K, respectively. Droplet vaporization rates and thermodynamic property histories were obtained. Results show that at low to moderate pressures the droplet remains spherical and retains a distinct temperature profile throughout the entire vaporization process. In contrast, at very high pressures, the droplet vaporizes in a could-like manner with vanishing surface tension. Droplet temperature histories show that the quasi-steady approximation is valid even when the environment pressure is above the critical pressure of either chemical species. In addition, the environment pressures required to cause the observed transition in vaporization behavior were well above the pure species＇ critical pressures, which confirms that many of the assumptions used in subcritical droplet evaporation analyses may be valid at pressures much higher than the critical pressure of oxygen. It was also found that significant concentration of the gas phase species was present in the droplet even at moderate pressures.