Ozone is

Allotropy is the property of an element to exist in more than one structural or molecular form while remaining chemically the same element -- the atoms involved are all of one kind; only their number per molecule or their arrangement differs. This is distinct from an isomer (a term for two forms that share the exact same molecular formula but differ in atomic arrangement), a compound (atoms of two or more different elements chemically combined), and an isotope (atoms of the same element with the same number of protons but a different number of neutrons, hence a different mass number).

Ozone and molecular oxygen are both built purely from oxygen atoms; they differ only in how many oxygen atoms make up one molecule (three in ozone versus two in O2) and in the resulting bent structure. Since both forms contain oxygen alone and merely arrange it differently, ozone is an alternate structural form of the same element -- the defining feature of an allotrope.

• an isomer of oxygen -- isomerism requires two forms to share the exact same molecular formula, differing only in how the atoms are arranged; ozone (three oxygen atoms per molecule) and O2 (two oxygen atoms per molecule) have different molecular formulas to begin with, so they cannot be isomers of each other.

• a compound of oxygen -- a compound needs atoms of two or more different elements chemically joined; ozone's molecule is made of oxygen atoms alone, so it cannot be a compound.

• an isotope of oxygen -- isotopes are variants of the same atom that differ in neutron count and mass number, not in how many atoms combine into one molecule; ozone differs from O2 in atom count per molecule, not in neutron count, so isotope misclassifies it.

So ozone is correctly classified as an allotrope of oxygen.