I know it sounds obvious, but from experience, the best way to get over a oneitis is to f*ck other girls and to have other activities in your life.
Go out and get some p*ssy.
Seriously.
Oneitis happens when you lose perspective and get too engrossed in 1 girl. You begin to idealise the girl because you're thinking about her all the time so your mind literally creates $hit. Your mind forms some perfect amazing version of her, with a perfect, amazing relationship which in real life, doesn't work out that way. It isn't good for you mentally to have your reality warped so badly then have your hopes completely crushed.
Plus the fact that you keep obsessing over her shows YOU ARE BORED. You don't have enough to do to keep your mind busy. That's not healthy at all.
You can see how a oneitis can therefore make you insane in some cases.
So...for your own sanity...go hook up with other girls. F*ck many different girls in a short space of time if you can. Also preoccupy yourself with sport, the gym, studies, socialising with friends etc.