So,
An oxygen atom is a single oxygen not bonded to anything. This should ring a bell for chemists, who intuitively know that natural oxygen is diatomic (bonded to another oxygen). An oxygen atom by itself is extremely reactive (can be called a radical) because its shell is not complete (only has 6 electrons versus 8 to satisfy the octet rule).
When oxygen forms compounds, it almost always becomes more stable than its atomic counterpart, because it almost always steals two (2) electrons from its partner to become O²⁻, a very stable (and octet-satisfied) ion.