How can a free radical have its unpaired electron on an oxygen atom?

In chemistry, a free radical is a molecule or atom containing at least one unpaired electron. While many radicals have the unpaired electron on a carbon atom, it is possible for the unpaired electron to reside on an oxygen atom.12

This occurs in several ways:

• Stable Oxygen-Centered Radicals: Certain organic compounds are structured to support an unpaired electron on an oxygen atom. A notable example is the 2,4,6-tri-tert-butylphenoxy radical.2
• Delocalization and Resonance: In some radical ions, such as semiquinones, the unpaired electron and a negative charge may be distributed across the molecule through resonance. In these cases, the electron is not fixed to a single atom but is delocalized, increasing the stability of the radical.2
• Transient Species: Highly reactive, short-lived radicals like the hydroxyl radical ($·\text{OH}$) also feature an unpaired electron on the oxygen atom.3

Additionally, some molecules are naturally diradicals, such as the oxygen molecule (${\text{O}}_2$), which contains two unpaired electrons in its antibonding orbitals.45