Electrons distribution of oxygen atoms in ozone
The ozone molecule is described with two resonant structures(as in figure) in which all the oxygen atoms respect the octet rule. The oxygen in the middle has a positive charge because has only 5 valence electron  and the oxygen with the single bond has a negative charge because it has 7 valence electrons. 
 In the real molecule both the bonds have the same length so there isn't one single bond and one double bond. Also both the terminal oxygen atoms have the same partial negative charge instead of being one neutral and one negative(-1). 
The picture below is more realistic of the situation: 



In the electrostatic potential map you can see that the two lateral oxygen are equivalent: