It sounds like Phlebitis, which is the Irritation of the inner wall of the vein, causing it to swell. The vein in this situation usually feels hardened under the skin like a cord. It's not that the oxycodone is clogging your veins, but rather the venous walls are swelling, preventing blood flow (or cutting it down significantly to a point where registering is impossible.) phlebitis can put you at a risk for blood clots, the best way to avoid this (while still continuing IV drug use)is to use a micron filter if you're going to inject pills, use a new syringe for each injection, and to use sterile cookers and water.
Might want to check out [url="http://www.bluelight.ru/vb/threads/440892-Injection-Complications-FAQ-and-Mega-Thread]this[/url] thread.