The O-O bond, which occurs in molecular oxygen (O2), is classified as a nonpolar covalent bond. This classification is due to the fact that both atoms involved in the bond are identical oxygen atoms, which means they have the same electronegativity.
In a covalent bond, atoms share electrons. When the two atoms are the same, as in the case of two oxygen atoms, they share the electrons equally. Unlike polar covalent bonds, where there is a difference in electronegativity between the two atoms leading to an uneven distribution of electron density, the nonpolar covalent bond allows for an even distribution because neither atom attracts the shared electrons more strongly than the other.
In summary, since the O-O bond involves two identical atoms sharing electrons equally, it is characterized as a nonpolar covalent bond.