Don't think there's much of a "rush" with if oxycodone. And oral is about 80% bioavailability so the increase in effects from dose in mg is really quite minimal.
If it is an IV Rush u are after I would look to other drugs or try combining the oxy solution with cocaine or some antihistamines like cyclizine or diphenhydramine. This is bad Hr advice I know. And you probably will be against it as you seem to be smart as far as injecing street drugs go.
You can get valoid (cyclizine lactate 50mg/ml) solution specifically for injection and that should give the oxy a "rush". Sorry I can't give you any more advice.
Oh you can get 2ml barrels and put brown needles on the top. The blue ones are really for IM injection or for groin deep big veins.