  The high frequency dynamics of fluid oxygen have been investigated by
Inelastic X-ray Scattering. In spite of the markedly supercritical conditions
($T\approx 2 T_c$, $P>10^2 P_c$), the sound velocity exceeds the hydrodynamic
value of about 20%, a feature which is the fingerprint of liquid-like dynamics.
The comparison of the present results with literature data obtained in several
fluids allow us to identify the extrapolation of the liquid vapor-coexistence
line in the ($P/P_c$, $T/T_c$) plane as the relevant edge between liquid- and
gas-like dynamics. More interestingly, this extrapolation is very close to the
non metal-metal transition in hot dense fluids, at pressure and temperature
values as obtained by shock wave experiments. This result points to the
existence of a connection between structural modifications and transport
properties in dense fluids.
