Final answer:
Oxygen has a positive oxidation number when combined with fluorine, the most electronegative element, which always has an oxidation number of -1. This is an exception to oxygen's usual -2 oxidation state.
Step-by-step explanation:
Oxygen will have a positive oxidation number when combined with fluorine. This is because fluorine is more electronegative than oxygen, which is typically seen with an oxidation number of -2, except in this special case as well as in peroxides where it has an oxidation number of -1. Fluorine, the most electronegative element, always has an oxidation number of -1, and it is the only element with which oxygen can have a positive oxidation number.
For example, in compounds like dioxygen difluoride (O2F2), oxygen has a positive oxidation state. This is an exceptional situation, as oxygen usually exhibits a negative oxidation state due to its high electronegativity, only surpassed by fluorine. Remember, the sum of oxidation numbers in a compound must balance out to zero, or the charge of the ion if it is a polyatomic ion.