How do you calculate the formal charge of O3?

The formal charge of the ozone molecule is zero. Its Lewis structures do present charge separation.

With simple considerations, there are 18 to distribute around the 3 oxygen atoms (24 electrons in total; 6 are inner core). Typically, a Lewis structure of ##O=O^(+)-O^(-)##, would be depicted. Going from left ##O## to right ##O## and including the 2 inner core electrons on each atom, there are 8, 7, and 9 electrons around each oxygen centre, resulting in formal charges of ##0##, ##+1##, and ##-1##, respectively. Of course, I can draw the other structure, but the Lewis structure has the same electronic formulation.

Since the central oxygen has 3 regions of electron , this molecule is bent.