We gots #3xx6*"valence electrons"# to distribute....and thus...
#O=stackrel(ddot)O^(+)-O^(-)#...and of course I could draw the other resonance isomers....#""^(-)O-""^(+)stackrel(ddot)O=O#.
For #O=stackrel(ddot)O^(+)-O^(-)# AS WRITTEN from left to right as we face the page, there are #8#, #7#, and #9# #"electrons"# associated with each oxygen atom, leading to FORMAL charges of #0#, #+1#, and #-1# (remember that I include here the two inner core electrons, the #1s^2# set that are not formally involved in bonding, and of course #Z=8# for the oxygen atom!). The number of valence electrons in the same direction are #6#, #5#, and #7#...
And given this electronic arrangement, the central oxygen is #sp^2-"hybridized"# with #/_O-O-O~=117^@# (the other doubly bound oxygen atom is also #sp^2-"hybridized"#). This is slightly compressed respect to with the regular bond angle of #120^@# due to the influence of the oxygen lone pair, the which is closer to the central oxygen atom, and exerts a repulsive influence on the bonding pairs.
