In the review, the spectral and photochemical manifestations of the catalytic effect of a weakly bound environment on photoprocesses involving oxygen in a gas, solutions, and on the surface are examined in historical sequence. The progress in an experimental study of the nature of supramolecular photochemistry of weakly bound oxygen complexes associated with the use of X–O2 van der Waals complexes generated in molecular beams as a model system is considered. A review of the results of studying supramolecular photochemistry in X–O2 complexes using the velocity map imaging of photofragments is given. The nature of the “dramatic” enhancement of UV absorption in weakly bound oxygen complexes and the mechanism of photochemical processes provided by the “enhanced” absorption are discussed. Available experimental data and conclusions on the nature of absorption bands appearing in weakly bound X–O2 complexes, which give rise to the formation of singlet oxygen and other photooxidation intermediates, are considered.