In the compound Ni2O3, nickel's oxidation number is oxidation number is -2.
and oxygen's

+3

Explanation:

The name of this compound would be nickel (III) oxide. Oxygen typically has a charge of -2. Given there are 3 oxygen atoms, that would equal a charge of -6. The nickel is the cation and must equal the charge of the anion, so must have a charge of +6. Since there are 2 nickel atoms, the charge on each must be +3.

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