Interpreting from phase diagram whether O2 will float or sink
Yes, we can do that.

In fact, it is only the sign of the slope that matters. Say, you stand right below the gas/liquid curve; you apply a little more pressure, and you step from gas to liquid. How does that correspond to Le Chatelier's principle? Well, just like that: the system responds to increased pressure by decreasing its own volume, hence the liquid must be denser that the gas (which we of course already knew from independent sources).

Guess what?

The same reasoning applies verbatim to the liquid/solid curve, since it is sloped in the same direction.

As to the intermolecular forces, I think the usual hand-waving along the lines of "less freedom $\to$ less motion $\to$ lower volume" will do, and even that is redundant. Having solid denser than liquid is normal. If anything, it is the opposite situation that would require an explanation.