2006 Chevrolet Trailblazer Car Been Blowing Lot White Smoke Exhaust Muffler

my car has been blowing a lot of white smoke/exhaust from the muffler every time i start it up. doesnt matter if it is first start of the day or not. after a couple of this my check engine light came on. i called onstar for a diagnostic and the advisor said the problem was an abnormal problem with the oil pressure. any ideas on what i am looking at when i take it in? the car isnt overheating or anything. it doesnt smoke when i accelerate. once i start to drive the exhaust dissaptes. thanks