My guess is that there are a lot of demonically "oppressed" people too. My understanding of demonic oppression is where the demons are outside the person, but constantly attacking. I think I had a year long experience with it myself. I was chatting on-line with a nasty atheist when a wave of gloom washed over me. It was very abrupt and unusual. I felt like the floor had dropped out from under me. For over a year I was preoccupied with heretical ideas that I couldn't get out of my mind. About a year and a half later, I felt the gloom lift as suddenly as it had come upon me... Don't want to go through that again.
It can be interior too. One of the methods of spell casting is ingestion. I'm sure Fr.'s talk was informative and interesting, but I get concerned when priests claim victims bring it on themselves. That is not true in all cases. Most of the saints suffered terribly. Think of the Ladder of Divine Ascent icon.
Also, there is a fine distinction between oppression and obsession which I've never seen well delineated.