Some oxidation numbers for common elements are unusual.

In hydrides, such as NaH, LiAlH₄, and BH₃, hydrogen has an oxidation number of -1. These substances tend to be good reducing agents; hydrogen prefers an oxidation number of 0 or +1.

In peroxides, such as H₂O₂, Na₂O₂, and BaO₂, oxygen has an oxidation number of -1. These compounds tend to be unstable, since oxygen prefers 0 or -2 as an oxidation number.

In superoxides, such as KO₂, oxygen has an oxidation number of -1/2. Superoxides are quite reactive.

In some compounds, the atoms appear to have fractional oxidation numbers:

In these cases, the substance can be thought of as being made up from atoms with different, whole-number oxidation numbers. So, in Fe₃O₄, we think of 2 Fe atoms at +3 and one Fe atom at +2. In NaN₃, we think of one nitrogen each at +1, 0, and -2. In S₄O₆^2-, we think of two sulfurs at +6 and two sulfurs at -1.