I think O and Cl will form covalent bonds since they are both non-metals and don't have a large enough difference in electronegativities to create an ionic bond.
I hope this helps. Let me know if anything is unclear.
As the electronegativity value of an element increases It is more likely to make an ionic bond. e.g. the metallic elements. On the moderate values of electronegativity, elements tend to make covalent bond.
Oxygen and chlorine will make covalent bond because the electronegativity difference between these two elements is 0,5 so there bond will be covalent in nature.
