  Oxygen exhibits structural phase transformation from non-metallic
$\epsilon$-O2 phase into metallic $\zeta$-O2 phase at pressure of P = 96 GPa
(Desgreniers et al 1990 J. Phys. Chem. 94 1117; Akahama et al 1995 Phys. Rev.
Lett. 74 4690). Metallic $\zeta$-O2 phase is a superconductor with transition
temperature of Tc = 0.6 K at P = 115-120 GPa (Shimizu et al 1998 Nature 393
767). In this paper we have performed analysis of temperature dependent upper
critical field, Bc2(T), for $\zeta$-O2 phase (P = 115 GPa) and show that this
highly-compressed phase of gaseous molecular element is unconventional
superconductor with the ratio of Tc to the Fermi temperature, Tf, in the range
of 0.009 < Tc/Tf < 0.108.
