What are the long term effects of a oxycontin addiction? is it possible to overdose even if someone has been using them for years ?

Many, and they can: The natural history of opioid abuse is escalation of dose, which is usually gradual but is sometimes quite rapid, until major toxic effects of opioids become problems - severe constipation, weight gain, sleep apnea, and overdose with the potential for prolonged coma, brain damage and death. The amount of money needed results in criminality, and switching to IV use and heroin is common.