What is the oxidation state of each individual carbon atom in C2O4^(-2)?

The oxidation number of carbon atoms in C2O2−4 is the same for both atoms

The oxidation number of oxygen is −2, therefore, assuming that the oxidation number of C is x,

then: 2x+4×(−2)=−2

⇒2x=+6

⇒x=+3