The formation of ${\mathrm{O}}_3$ from ${\mathrm{O}}_2$ is

The reaction of the formation of ozone from oxygen is as follows-

$3{\mathrm{O}}_2\rightleftharpoons 2{\mathrm{O}}_3 \mathrm{\Delta H}=+284 \mathrm{kJ}$

Since enthalpy is + this means that the reaction is endothermic and also reversible.