Hi Ube, Only a couple of options for oil in a cylinder; it`s either coming up from below or it`s coming down from above. Check the piston rings aren`t damaged and that the ring gaps are not aligned allowing oil to blow up and compression to escape. Check condition of valves and guides, some cylinder heads have seals on the valves.
Just a thought, are you sure it was oil? A leaky injector could dribble diesel onto the piston.
Bob