The collisions between two oxygen molecules give rise to O₄ absorption in the Earth atmosphere. O₄ absorption is relevant to atmospheric transmission and Earth's radiation budget. O₄ is further used as a reference gas in Differential Optical Absorption Spectroscopy (DOAS) applications to infer properties of clouds and aerosols. The O₄ absorption cross section spectrum of bands centered at 343, 360, 380, 446, 477, 532, 577 and 630 nm is investigated in dry air and oxygen as a function of temperature (203–295 K), and at 820 mbar pressure. We characterize the temperature dependent O₄ line shape and provide high precision O₄ absorption cross section reference spectra that are suitable for atmospheric O₄ measurements. The peak absorption cross-section is found to increase at lower temperatures due to a corresponding narrowing of the spectral band width, while the integrated cross-section remains constant (within <3%, the uncertainty of our measurements). The enthalpy of formation is determined to be ΔH²⁵⁰ = −0.12 ± 0.12 kJ mol⁻¹, which is essentially zero, and supports previous assignments of O₄ as collision induced absorption (CIA). At 203 K, van der Waals complexes (O_2-dimer) contribute less than 0.14% to the O₄ absorption in air. We conclude that O_2-dimer is not observable in the Earth atmosphere, and as a consequence the atmospheric O₄ distribution is for all practical means and purposes independent of temperature, and can be predicted with an accuracy of better than 10⁻³ from knowledge of the oxygen concentration profile.