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Ball-and-stick model of O8	Part of the crystal structure of ε-oxygen

Octaoxygen, also known as ε-oxygen or red oxygen, is an allotrope of oxygen consisting of eight oxygen atoms. This allotrope forms at room temperature at pressures between 10 and 96 GPa.^[1]

As the pressure of oxygen at room temperature is increased above 10 gigapascals (1,500,000 psi), it undergoes a dramatic phase transition to the 𝛆-phase allotrope. Its volume decreases significantly, and it changes color from sky-blue to deep red.^[2][3] This ε-phase was discovered in 1979, but the structure has been unclear. Based on infrared spectroscopy, researchers assumed in 1999 that this phase consisted of O₄ molecules in a crystal lattice.^[4] However, in 2006, it was shown by X-ray crystallography that this stable phase is a rhomboid O₈ ((O₂)₄) cluster consisting of four O₂ molecules.^[5][6][7]

In this phase, it exhibits a dark-red color, very strong infrared absorption, and a magnetic collapse.^[8] It has been shown to have a monoclinic C2/m symmetry, and its infrared absorption behaviour was attributed to the association of oxygen molecules into larger units. At 11 GPa, the intra-cluster bond length of the O₈ cluster is 2.34 Å, and the inter-cluster distance is 2.66 Å, both longer than the 1.2 Å bond-length in the oxygen molecule (O₂). It is stable over a very large pressure domain, from 10 GPa to 96 GPa. Above 96 GPa, ζ-oxygen forms which is metallic and superconductive; ε-oxygen is non-conductive.^[1][2][9]

The formation mechanism of the O₈ cluster found in the work is not clear yet, and the researchers think that the charge transfer between oxygen molecules or the magnetic moment of oxygen molecules has a significant role in the formation.^[1]

Liquid oxygen is already used as an oxidant in rockets, and it has been speculated that octaoxygen could make an even better oxidant, because of its higher energy density.^{[10][need quotation to verify]}