Of course, everyone has their own reasons for believing whether or not OP might have known Reeva was in the toilet, or whether he simply thought it was an intruder.
I have personally found no other explanation that satisfies ME, other than he knew, 100%, it was Reeva in that toilet. This, as I've said, is what I believe. Others may see it differently of course. Some of the key reasons I have, that swayed me, are as follows:
A) all the testimony from strangers that stated they heard arguing and also screaming from a woman.....before the time of the killing. The neighbor seeing the lights on in the bathroom and movement in front of the window of a person...before the killing.
B) when OP gets the gun - he would have warned her. Simple...he would have warned her + he would have made sure she was awake. Time had passed from asking him in the middle of the night after being awoken, "can't you sleep baba?" ....after that OP got up, brought in 2 fans, closed doors, closed curtains, was dealing with annoying stereo light, and all while on his stumps. Since no talking or help offered by Reeva - very likely she rolled over + went back to sleep. So he would have made sure she was awakened + warned her of what "he thought he heard" (although he simply hears a window opening, nothing more yet - he says he is certain he knows it's an intruder). He is inches from her, he would have warned his loved one, and made sure she was awake.
C) I don't think the "ugly green key" in the bathroom door was something left there day to day. I believe it was kept out of sight in one of the drawers. As no need for a single man to have a key attached to an ugly plastic green fob hanging from the master bathroom door of his modern, expensive, sleek bachelor pad home. Just silly to think that for a second. He is not going to need it or use it. It's his bathroom. And only other person using that bathroom...ever, are going to be female guests/girlfriends. And OP will not anticipate them ever needing/wanting it. So in the drawer it goes. Which means...OP got it out AFTER he shot Reeva, to open the door. (I also think if the key was left in the door on a regular basis, it would definitely NOT have that green plastic fob hanging from it. It would be just the key, left in the door for someone if needed. The plastic green fob is attached so a "spare key" doesn't get lost, like when it's thrown in a drawer. IF HOWEVER, THE KEY WAS IN THE DOOR, I believe it was because Reeva reached in the drawer and grabbed it, to lock herself in, safe from OP, or so she thought.)
D) I believe he was mad & scaring Reeva. And had the gun in his hand while doing so. Why was he mad? I don't know, could be a number of reasons which we will never know. I believe Reeva, scared...ran to the bathroom. And eventually said she was going to call the police. OP lost it at this point and fired into the toilet to stop her from doing this. He was not about to let his celebrity status, his career, his idolization/hero status by So. Africa be jeopardized by this night...Reeva exposing his crazy gun welding actions. He couldn't allow that. Oscar being Oscar...the narcissist he was....wasn't too worried about escaping punishment. He knew he would ALWAYS be believed with whatever story/crap he came up with. He had always been protected before. But NOT if Reeva was there to tell her 1/2 of the story, that might be a problem. It could potentially ruin/damage his career. A lot.
(Of course this is ALL speculation on my part)
E) the moving of Reeva...he even said in his ITV interview, he knew right away she was dead. So why move her downstairs? Because it was so important to alter the scene. And because he wanted to get Reeva out of there, hopefully someone else take her to the hospital, giving him more time to clean things up, come up with answers, and to hide/alter anything that might conflict with this intruder story....maybe fetch the pants thrown out the window, unpack her bags, etc.
He is lying about everything IMO. His "story" doesn't hold water.