Spread onion in a slow cooker. Season pork roast with salt and pepper. Place on top of onion. Pour cola over roast.
Cover and cook on low setting for 6 to 8 hours, until very tender.
Remove roast from slow cooker; discard cooking juices and onion.
Return roast to slow cooker; shred with 2 forks. Add BBQ sauce and mix well.
Serve pork mixture on buns.