In Na2O oxidation state of O is -2.
Explanation
The oxidation state of an atom (sometimes also referred to as the oxidation number) in a chemical compound gives insight into the number of electrons lost it and, therefore, describes the extent of oxidation of the atom.
Na2O
O is more electronegative than Na, hence, Na is given +1 oxidation state and O shows oxidation state as given below
Let us assume that the oxidation state of oxygen in Na2O is x
2(+1)+ (x)=0
+2 +x= 0
x=-2
In Na2O oxidation state of O is -2.