Assign charges to each of the ligands and to the central cation: OsO4
Os is a transition metal in period 6. It has 8 valence electrons (6s2, 5d6) and forms double bonds with each oxygen resulting in a 16 electron complex.
Conventionally, the metal will have an oxidation number of +8
x + 4(-2) = 0
x + (-8) = 0
Thus the charge on Osmium should be 8+ and for each oxygen is 2-.
However, Os does not possess an 8+ charge because the Ionization Energy required to produce this charge (8+) far much exceeds the energies available in normal chemical reactions, and that is why the bonding is largely covalent.