In various models of the chemistry of interstellar grains, solid O2 is formed by accretion as well as by surface reactions. In dense molecular cloud models, at a later stage of evolution of an interstellar grain, solid O2 becomes a major grain mantle constituent at the expense of water ice abundance. If molecular oxygen is embedded in a “dirty” ice” matrix, the forbidden fundamental vibration of O2 at 1550 cm−1 may become observable.