O2

O2, O-2, o2, or similar orthography may refer to:

==Science and technology== Dioxygen (), the common allotrope of oxygen Oxide (), an ion Superoxide (), an ion Dioxygenyl (), an ion Doubly ionized oxygen (), an ion O(2), the 2-dimensional orthogonal group in group theory O2, an EEG electrode site according to the 10–20 system SGI O2, a Unix workstation computer O2, a class of O-type star Orbital O2, a tidal power turbine in Orkney, Scotland -O2, a compiler optimization tag