I think that the cat would need an enema. If the cat has impacted feces to the point that it is now mega colon, I do not think that any remedy given by mouth will work. I think it will have to given from the other end. My two cents worth and I could be wrong but this is my gut instinct, no pun intended.
I would be taking to cat to the vet in the morning for sure.
I would NOT attempt to give the cat a home enema. Neither would I try and give it any more to eat and I would not try and get any olive oil down the cat at home.