Spray 5- to 6-quart slow cooker with cooking spray. In cooker, place onion wedges. Sprinkle with half each of the salt, pepper and oregano. Drizzle with oil.
If beef roast comes in netting or is tied, do not remove; place beef on onions. Sprinkle beef with remaining salt, pepper and oregano.
Cover; cook on Low heat setting 8 to 9 hours. Remove beef to cutting board; keep warm. Strain onions and cooking liquid into 2-quart saucepan; reserve onions.
In small bowl, stir water and flour until well blended; stir into cooking liquid. Cook over medium heat 3 to 5 minutes, stirring frequently, until gravy is thickened and bubbly.
Remove netting from beef if necessary. Cut beef into slices; place on serving platter. Surround beef with onions. Serve with gravy; garnish with fresh oregano.