Pressure and density are only proportional for ideal gases. At sea level pressure that's a good approximation, at 1000 times that pressure it is not. It's in a supercritical state, which is a state between a gas and a liquid. This page says nitrogen has a density of 0.3 g/cm^3 at the critical point. I don't find data for 1000 times the atmospheric pressure, but it's likely the density doesn't exceed the density of liquid nitrogen (~0.8 g/cm^(3)), so it would still rise. The 20% oxygen content shouldn't change that, even though liquid oxygen has a higher density than water.