In the present study, bubble nucleation in liquid oxygen where impurities (nitrogen molecules or helium ones) are dissolved is investigated using molecular dynamics simulation. When the mole fraction of impurities is 0.05, bubble nucleation mechanism is fundamentally different ; vaporization in homogeneous bulk makes a bubble in the case of nitrogen-dissolved liquid while phase separation of impurities makes a nucleus in the case of helium-dissolved liquid. The effect of fluctuation to bring a local void, which can grow to be a bubble, is stronger in the case of helium-dissolved liquid under the lower mole fraction (0.01) than in the case of nitrogen-dissolved liquid under the higher mole fraction (0.05). From these results, we found that helium molecules have much stronger action to raise bubble formation pressure compared with nitrogen. In this paper, the kinetically-defined critical nucleus, which is very important factor to evaluate the nucleation mechanism qualitatively, is also estimated through the calculation of size change rate of each nucleus.