The Oval Portrait has the characteristic of being a story nested within another story. The story is of a wounded man who ended up in a chateau. The Chateau's gallery has apparently a bunch of great artworks, for which there is a boo that explains what each one is. The oval portrait caught his attention, and he reads about it. In comes the nested story, which is how the painting came to be.
In the nested story, we learn that apparently the artist who painted the Oval Portrait was trying to get his girlfriend portrayed, but in the process became obsessed with the work, and deviated completely from her portrait, and ended up painting what seems to be a horrible reflection of his soul in turmoil, and we also learn that he found after that spell of craziness that his girlfriend was dead.
So, as with all of Poe's works, there is a lot left for personal views: 1. Did the painter kill the girlfriend in the middle of his insanity? 2. Was he already insane, or did "art" gave him such an overwhelming sensation that it drove him mad 3. Did the girlfriend kill herself upon seeing the artist go nuts? 4. What ever happen to the protagonist to be wounded like that in the first place?
Hope this helps!