What is ionic charge of oxygen?

O2-

The ionic charge of something can be determined by it's place in the Periodic Table.

Elements in groups 1-3 have a positive charge the same as their group number,

e.g. Magnesium (Mg) Is in group 2 so its charge is Mg2+

Elements in groups 5-7 have a negative charge equal to their group number -8

e.g. Oxygen is in group 6 (6-8=-2) So its charge is O2-

The noble gases (Group 0) do not have an ionic charge.