No, oxycodone's metabolic half life is constant at 3-4.5 hours according to Wikipedia. Oxycontin's extended release formulation just means that doses of oxycodone will be dispersed over 8 hours, and thus each dose would have its own half life.
Luckily Oxycontin's extended release means that you will have progressively smaller doses to withdraw from and each dose will have its own half life (if you are taking it as directed) whereas with the Roxycodone the withdrawal and half life would occur at the same time.