What are the various forms in which oxygen is available?

(N/A) $(i)$ In the atmosphere,oxygen is found in the elemental form to the extent of $21 \%$.
$(ii)$ In combined form,it occurs as $CO_{2}$ both in the earth's crust as well as in the air.
$(iii)$ In the crust,it is found as the oxides of most metals and silicon,and also as carbonate,sulphate,nitrate,and other minerals.
$(iv)$ It is also an essential component of most biological molecules like carbohydrates,proteins,nucleic acids,and fats (or lipids).