Why do the pennies in the pre/post 1982 penny experiment sometimes dissolve completely?
Your conclusion about formation of the Copper Oxide is correct, but not about the reaction with atmospheric oxygen.

The origin of the Oxygen is from the hydrochloric acid solution. The solution does not mean is pure HCl , it means is a dissolve in water.  There is a redox reaction with water

I leave to you to find the redox reaction.