Are photolysis reactions considered exothermic or endothermic?
In general it could be either. In your example you will have to work out the heat of formation of ozone and of O atoms (presumably it is an endothermic reaction). In general what the photon is doing is providing a way to overcome an activation barrier in the ground state by opening up a pathway from an excited state to the products. This last step is generally exothermic.

The enthalpy released in the second reaction is less than the photon energy by the difference in energy of the photon and that of products, an oxygen atom in your case as the heat of formation of O$_2$ is zero. 

The sketch gives the general idea, I hope, for an endothermic reaction :)



Notes: The barriers may also be called transition states. There may be no barrier in the excited state. The crossing from the excited state potential to the product may be controlled by Landau-Zener behaviour or be a conical intersection. The far right upper line (not labelled) is the potential of the excited state of the product