Typically we would utilize an alkali metal superoxide, i.e. #K^(+)O_2^(-)# or #Na^(+)O_2^(-)#...and as always, in a compound ion such as #O_2^(-)#, or #SO_4^(2-)#, or #NO_3^(-)#, the sum of the oxidation numbers of the constituent elements equals the charge on the ion....
We deal with sulfate, and nitrate, first to establish the principle...
#SO_4^(2-); S(+VI) + 4xxO(-II)=-2_"the ionic charge of sulfate"# as required....
#NO_3^(-); N(+V) + 3xxO(-II)=-1_"the ionic charge of nitrate"# as required....
And now for superoxide....we gots #O_2^(-)#, and for the Lewis structure, we have to distribute #13*"valence electrons"#...
And so ....
The Lewis structure as written features a neutral radical (lone electron) oxygen on the #"left hand side"# as we face the page....but the LHS oxygen claims 6 electrons (the single electron, the 2 lone pairs, and a single electron from the #O-O# bond... to give, with the 2 inner core electrons, EIGHT electrons in total, the which electrostatically balances the 8 nucular protons present in the nucleus of the oxygen atom. And so this centre is NEUTRAL. Capisce?
On the RHS of the diagram, we gots 7 valence electrons, 6 from the lone pair, and one from the #O-O# bond, and two inner core, to give 9 electrons, and a consequent charge of #-1#... Do you follow? And these charges specify, at least for the given Lewis structure, oxidation states of #0#, and #-I#.
Of course, the oxygens are treated as equivalent beasts...certainly, we could push electrons around, and so EACH assume an AVERAGE CHARGE of #-1/2# as required.
I don't know at which level you are at...but this is probably 2nd year inorganic/organic...
Just to add that for the purposes of assigning formal charge, we can go back to very old ideas that we learn when are introduced to bonding. In a covalent bond, electrons ARE SHARED between nuclei. An ionic bond is between a formal anion, and a formal cation, and thus involves the prior TRANSFER of electron.
If we take say methane, with FOUR #C-H# covalent bonds, we split this up to give #4xxdotH#, and a carbon atom with 4 valence electrons. The hydrogen ATOMS are neutral, because they have a positive nuclear charge, and also an electronic charge. And hence the hydrogens are neutral.
Likewise, the carbon atom claims 4 electrons from the four #C-H# bonds, and has 2 inner core electrons (i.e. formally the #1s^2# electrons. And so each carbon has 6 electrons, and there are 6 positive nuclear charges. The carbon atom in methane is also thus electronically neutral.
Confused yet?
