There are a few types of strokes...
Most commonly there are bleeds, and there are clots.
Bleeds are when a vessel in your brain somewhere bursts.
CVA's involving clots happen when the fatty deposits on the inside of your veins and arteries, called atherosclerosis, break free from the wall of your inner vein and travel into an area of your brain where they get stuck. This blocks any oxygenated blood from traveling past the blockage and causes the tissue downstream to begin to die.
SO, its not that the blood traveling into that part of your brain has no Oxygen in it, its that there is no freshly oxygenated blood getting past the block in that brain vessel.
I hope that helps you understand the process with regards to CVA's.
As far as opiates increasing the risk of someone suffering from a CVA while working out I can't answer that part for you, sorry and good luck