Heat oil in large skillet over medium-low heat until hot. Add roast; brown on all sides. Drain and discard excess fat. Transfer roast to 4 1/2 quart CROCK-POT slow cooker.
Add potatoes, carrots, onions, celery and tomatoes with juice. Season with salt, pepper and oregano to taste. Add enough water to cover bottom of CROCK-POT slow cooker by about 1/2 inch. Cover, cook on LOW 8-10 hours.
Transfer roast and vegetables to serving platter. To make gravy, whisk together cooking liquid and flour in small saucepan. Cook and stir over roast and vegetables.