The o-to-o-to-o bond angle in an ozone (o3) molecule is not exactly 120° because

There is an extra pair of electrons bound to the central oxygen. This set of electrons repels against the other electrons surrounding the outside oxygen molecules, compressing the bond angle more than if another atom were present in its place. The actual angle size is around 116 degrees, noticeably smaller than the 120 degrees present in a trigonal planar structure.