Oxygen and ozone are allotropes. Allotropy is the phenomenon in which an element can exist in different forms, known as allotropes, with different physical and chemical properties. Oxygen and ozone are both forms of oxygen, but they have different molecular structures and properties.

Option 1: Allotropes - This is the correct option. As mentioned above, oxygen and ozone are examples of allotropes.

Option 2: Isomers - Isomers are molecules with the same molecular formula but different structural arrangements. This is not applicable to oxygen and ozone as they have different molecular formulas.

Option 3: Isotopes - Isotopes are atoms of the same element with different numbers of neutrons. Oxygen and ozone are not isotopes since they have different molecular structures.

Option 4: Isobars - Isobars are atoms or ions with the same atomic mass but different atomic numbers. This is not applicable to oxygen and ozone as they are not atoms or ions.

In conclusion, the correct answer is option 1: oxygen and ozone are allotropes.