  Unusual metallic states involving breakdown of the standard Fermi-liquid
picture of long-lived quasiparticles in well-defined band states emerge at low
temperatures near correlation-driven Mott transitions. Prominent examples are
ill-understood metallic states in $d$- and $f$-band compounds near Mott-like
transitions. Finding of superconductivity in solid O$_{2}$ on the border of an
insulator-metal transition at high pressures close to 96~GPa is thus truly
remarkable. Neither the insulator-metal transition nor superconductivity are
understood satisfactorily. Here, we undertake a first step in this direction by
focussing on the pressure-driven insulator-metal transition using a combination
of first-principles density-functional and many-body calculations. We report a
striking result: the finding of an orbital-selective Mott transition in a pure
$p$-band elemental system. We apply our theory to understand extant structural
and transport data across the transition, and make a specific two-fluid
prediction that is open to future test. Based thereupon, we propose a novel
scenario where soft multiband modes built from microscopically coexisting
itinerant and localized electronic states are natural candidates for the
pairing glue in pressurized O$_{2}$.
