I've never gone as far as you with the name thing as in actually buying stuff with her name on it, decorating etc (way too superstitious for that as well as a git DH), but DD1 was pretty definitely Sarah for months. When she was born there was just no way she was a Sarah, so she became something else.

I have a pang every now and then that I didn't push harder for a different name for one of my girls, so I would definitely say if your boy comes out and you're not feeling the Oliver (which I love by the way), then choose something else