Octaoxygen, also known as õ-oxygen or red oxygen, is an allotrope of oxygen consisting of eight oxygen atoms. This allotrope forms at room temperature at pressures between .
As the pressure of oxygen at room temperature is increased above , it undergoes a dramatic phase transition to the -phase. allotrope. Its volume decreases significantly, and it changes color from sky-blue to deep red. This õ-phase was discovered in 1979, but the structure has been unclear. Based on infrared spectroscopy, researchers assumed in 1999 that this phase consisted of molecules in a crystal lattice. However, in 2006, it was shown by X-ray crystallography that this stable phase is a rhomboid () cluster consisting of four molecules.
In this phase, it exhibits a dark-red color, very strong infrared absorption, and a magnetic collapse. 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 , the intra-cluster bond length of the cluster is , and the inter-cluster distance is , both longer than the bond-length in the oxygen molecule (). It is stable over a very large pressure domain, from to . Above , ö-oxygen forms which is metallic and superconductive; õ-oxygen is non-conductive.
The formation mechanism of the 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.
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.