Show how the total charge sodium oxide is zero

First, the symbol for sodium oxide is Na₂O

Each Na (sodium) has a charge of 1+, and each O has a charge of 2 -:

Na₂¹⁺O²⁻

There are two Na's, however, and each one is 1+, however, so the Na₂ has a total charge of 2+. Because of this, the 2 + from the 2 Na's and the 2 - from the O cancel each other out to make 0.