immediate response, 'it IS w/d related'. I am an oppiate addict, and always dose before bed (as to try to make me NOT sweat). I recently (a year ago..., the time warp of addiction) checked my mattress and it has a big yellow stain on my half of the bed (I now sleep alone, yay for another effect of addiction, being alone).
So, you can still sweat at night, while dosing at night before you sleep. How are you taking your Oxy, and are you taking IR or ER oxy? If IR, regardless of method: with regular use you'll be sweating 4+ hours after taking it. Esp if taking it other than orally.