Question

When does oxygen exhibit $+ 2$ oxidation state?

Answer 1

A positive oxidation state is an indicator of the fact that the element is bonded to a more electronegative element. The electronegativity of elements decreases on going down in a group and increases on going left to right in a period

Complete answer:
Oxidation number is a hypothetical number that is used to measure the number of electrons that an atom contributes in a bond. Though the magnitude of oxidation number deals with the number of electrons, its sign (positive or negative) is dependent on the polarity of the bond.
A polar bond always contains an electropositive and an electronegative atom. The electronegative atom has a higher tendency of pulling the shared pair of electrons of a covalent bond towards itself. The electronegative element gets a negative oxidation number and the electropositive element gets a positive oxidation number.
The only element present in the modern periodic table that has a higher electronegativity than oxygen is fluorine. Since fluorine is capable of contributing a single electron due to its electronic configuration, its oxidation number is $- 1$ . When two fluorine atoms get bonded to a single oxygen atom, the shared pair of electrons move towards fluorine resulting in a $+ 2$ oxidation state.
Hence, oxygen exhibits $+ 2$ oxidation state in $O{F_2}$ molecules.

Note:
The oxidation state or oxidation number can be assigned in a covalent molecule or a complex. The oxidation number and its sign indicates the direction in which the electrons are pulled in a bond and should not be compared with actual charge. The $+ 2$ oxidation state does not mean that the oxygen atom completely loses its two electrons to another atom.