If the element is oxidized, then it's oxidation number would increase.
Let's say we have the following reaction.
2H2O --> O2 + 2H2
To the left, the oxidation number of O is -2, as it is bonded to two H, which is always H+. To the right, we have O2, and all gases have a oxidation number of 0 (zero). We say that O has been oxidized.
