Species exhibiting diamagnetic behavior:
- O2⁺
- O2
- NO
- O2^2-

Explanation:
Diamagnetic behavior is characterized by the presence of paired electrons in the molecular orbital, resulting in a weak repulsion when exposed to a magnetic field.
- O2⁺: O2⁺ has one less electron compared to O2, leading to the presence of unpaired electrons. Therefore, it exhibits paramagnetic behavior rather than diamagnetic behavior.
- O2: O2 is a paramagnetic molecule due to the presence of two unpaired electrons in its molecular orbital, causing it to be attracted to a magnetic field.
- NO: NO is a paramagnetic molecule as well, as it has an odd number of electrons that cannot all be paired in the molecular orbital.
- O2^2-: O2^2- has two additional electrons compared to O2, resulting in the pairing of all electrons in the molecular orbital. This leads to diamagnetic behavior, where the molecule is weakly repelled by a magnetic field.
In summary, among the given species, O2^2- exhibits diamagnetic behavior due to the presence of paired electrons in its molecular orbital, leading to a weak repulsion in a magnetic field.