Answer – The oxidation number of oxygen is –2.

Explanation: 

Oxidation number, or oxidation state, refers to the total number of electrons an atom needs to gain/lose to complete its octet and form a bond. This number may vary for the same element in different compounds.

When an atom loses electrons, its oxidation number is preceded by a positive sign; when it gains electrons, the oxidation number is preceded by a negative sign.

The atomic number of oxygen is 8. Thus, it has 8 electrons and its electron configuration is 1s²2s²2p⁴.

An oxygen atom needs 2 more electrons to complete its octet. Since this would mean the addition of electrons, a negative sign must be included. Thus, the oxidation number of oxygen is –2.

However, there are three situations where the element has a different oxidation number:

1. When oxygen occurs in a compound with fluorine

Example: In OF₂ (oxygen difluoride), the oxidation state of oxygen is +2. This is to balance the oxidation number of fluorine (–1 x 2 atoms = –2) in the compound.

2. When oxygen occurs in a peroxide

Example: In H₂O₂ (hydrogen peroxide), each oxygen atom has an oxidation state of –1. This is to balance the oxidation number of hydrogen (+1 x 2 atoms = +2) in the compound.

3. When oxygen is present as a free element

Much like other free elements, O₂ (oxygen molecule) has an oxidation number of zero.

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