  Absorbing UV radiation, ozone protects life on Earth and plays a fundamental
role in Earth's temperature balance. The formation of ozone occurs through the
ternary recombination reaction: O$_2$+O+M $\rightarrow$ O$_3$+M, where M can be
N$_2$, O$_2$ or Ar. Here, we developed a theoretical approach capable of
modeling the formation of ozone molecules in ternary collisions, and applied it
to the reaction with M=Ar because of extensive experimental data available. The
rate coefficients for the direct formation of O$_3$ in ternary collisions
O+O$_2$+Ar were computed for the first time as a function of collision energy,
and thermally-averaged coefficients were derived for temperatures 5-900~K
leading to a good agreement with available experimental data for temperatures
100-900~K. The present study shows that the formation of ozone in ternary
collisions O+O$_2$+Ar at temperatures below 200~K proceeds through a formation
of a temporary complex ArO$_2$, while at temperatures above 1000~K, the
reaction proceeds mainly through a formation of long-lived vibrational
resonances of O$_3^*$. At intermediate temperatures 200~K-1000~K, the process
cannot be viewed as a two-step mechanism. In addition, it is found that the
majority of O$_3$ molecules formed initially are weakly bound.
