Double Covalent.
Step-by-step explanation
O₂ exist as molecules. The subscript "2" indicates that there are only two oxygen atoms in each molecule. That rules out the possibility of ionic bonds. Ions would combine to form ionic compounds. Those compounds would contain a very large number of atoms.
Oxygen is a group 16 (VIA) element. There are six valence electrons in each oxygen atom. Each atom is two electrons away from an octet of eight electrons. An atom shares one electron with another for each covalent bond that it forms with another atom. Each oxygen atom needs two electrons and would form two covalent bonds.
There would be two covalent bonds in each O₂ molecule. The result is a double covalent bond.