Combination reactions are exothermic than howcome oxidation reaction of Zn+O2 gives zno which is an endothermic reaction.

Dear Student,
All combination reactions are not exothermic the can also be endothermic.
In case of combination reaction of ZnO for the combination of Zn and O we need to break the stable O=O double bond, for breaking this bond we need the energy, therefore, it is endothermic.
Regards