Is the synthesis of ozone, #O-3#, from oxygen, #O_2#, an example of an exothermic or endothermic reaction?
The reaction as written is ENDOTHERMIC, and it is accomplished in the upper atmosphere by the impact of photons on the dioxygen molecule, i.e.
The first reaction is bond-breaking, and is certainly endothermic, and the second reaction, bond-making, is exothermic. The balance is towards bond-breaking, i.e. the reaction as written in the intro is endothermic.