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The lowest excited electronic state of the O2molecule, a1Δg, the “singlet oxygen”, is of utmost importance for photochemistry and photobiology. For O2trapped in silica glass, the lifetime of this state and the associated a1Δg→ X3Σg–photoluminescence (PL) is the longest known for O2in any condensed medium at room temperature. We studied the temperature dependence, decay kinetics, and polarization anisotropy of this PL with 1064 nm excitation to the a1Δg(v= 1) state as well as with excitation to higher energies. PL at this excitation shows nonzero polarization anisotropy at 295 K, which increases with cooling to 14 K. At variance, excitation to higher energies yields depolarized PL. Polarization data indicate weak electric dipole character of the emission of the spin- and parity-forbidden a1Δg→ X3Σg–transition, enabled by O2–SiO2cage interactions. Density functional theory calculations indicate that at low temperatures the rotation of O2is partially or fully frozen even in large silica voids. As the temperature increases, PL is increasingly depolarized by libration movement of O2molecules. Analysis of O2optical absorption in optical fibers allows one to obtain the absorption cross sections of X → a and X → b transitions of O2in SiO2glass and to evaluate both radiative and nonradiative rates of a → X luminescence.