Season the roast with salt and pepper. Brown it in the oil for 4 minutes per side and transfer it to a 6-quart slow cooker over the sliced onion.
Whisk the cola, tomato sauce, vinegar, tomato paste, Worcestershire, mustard, paprika, garlic powder and cayenne.
Pour the sauce over the beef, cover, and cook on low for 8 hours until the meat shreds easily.
Transfer the roast to a rimmed pan and shred it, discarding large pockets of fat. Pour the cooking liquid into a saucepan and skim it.
Boil the sauce for 8 to 12 minutes until it coats a spoon. Toss about 1 1/2 cups with the shredded beef and serve the remaining sauce at the table.