I used to be a drug addict and I have injected
oxycodone before. The reason this is possible is because the Name Brand Oxycodone (Oxycontin) has a water soluble buffer in it. This means that you can crush the pill and mix it with water, which then dissolves allowing you to place it in a needle and inject it. The overdose problem arises when you think that you need more than you do to get "high". Because Opiates can cause a dose dependent tolerance, knowing exactly how much will get you high and how much will kill you, is not all that easy. I hope that answers your question.
To everyone reading this: PLEASE DO NOT use any of this information to abuse drugs, I have been down that road and after causing my heart to stop multiple times and being shocked back to life, I finally learned that drugs are not the answer to all of life's problems.