How is O2 transformed into O3?

By Free Radical formation in the Upper Atmosphere, where oxygen molecules (O2) are subject to much stronger solar Radiation.

By Solar radiation any one oxygen molecule is split into two free radicals.

Here is the reaction eq;n

O2 == Solar radiation ==> O + O

NB ' O ' is an oxygen free radical.

This radical then reacts with another oxygen molecule (O2) to form O3 (Ozone)

O + O2 = O3

Structurally O2 is O=O , a double bond between the two oxygen atoms.

O3(Ozone) is a triangular shaped molecule, with any one oxygen atom having two bonds, one each to the other two oxygen atoms.

Ozone stops the worst effects of solar radiation reaching the lower level atmospheric oxygen molecules, and so they do not 'free radicalise'. Ozone also stops the worst effects of solar radiation reaching ground level; so the biOSphere ( animals(humans) plants etc) are protected from the worst effects of solar radiation, such as Cosmic Rays, or X-Rays. We receive light, the UV light and the infra red light.