Oil drain plug gasket sealed to oil pan

I’ll jar them loose putting a slim, flat-bladed screw driver at the junction of washer and oil pan, give it a tap with a hammer.

and yeah looks to be the washer; shouldn’t take more than a light tap.

 

Happens to me pretty much every oil change. I generally just poke the tip of a small screwdriver right up through the hole at a shallow angle so it's between the oil pan threads and the gasket on the opposite side, then push gently by hand on the screwdriver handle to pop the gasket off. Gently is the watchword, no need to go ballistic and nick the threads.