In this equation 2mg+o2=2mgo what is the subscript of the oxygen molecule

The subscript of the O₂ molecule is the small "2" at the lower right-hand corner of the "O".

Explanation:

A subscript is a letter or a number that is written below the line.

In chemistry, a subscript after an atomic symbol tells you how many of those atoms are in the unit.

Thus, the subscript in O₂ tells you that there are two oxygen atoms in a molecule of oxygen.