Im going to make a guess at the sequence of events that occured and we'll see how close I am.
Oscar and his girlfriend had a huge argument, he threatened her (remember the police have already been called previously on domestic issues between the couple) and so she ran to the bathroom and locked the door. Incensed he banged against the door trying to break in, she was on the other side, crouched down, her weight pressed against the door with her hands on the handle to prevent him getting in. Oscar is so mad, he grabs his gun and starts shooting at the door handle to try and get in. This would explain the wounds to the hand and the head, as crouched down against the door, her head would be around where the handle of the door is.
You watch, I reckon Im pretty accurate ...... lets see hey?