You took 3/4ths, so, you had roughly 22.5mg of Oxycodone? And you want to take 15mg on top of that, even though the dose was three hours ago?
Tolerance is important here. How often do you take opiates? 15mg would probably produce subtle effects but it isn't gonna be like the initial dose you took. It might be a little too soon, that's why most labels on prescriptions have a 4-6 hour waiting period before redosing. Maybe potentiate it if you're that interested in taking it. Or...
Save it for a rainy day?