Sprinkle brisket evenly with 3/4 teaspoon salt and 1/4 teaspoon freshly ground black pepper. Heat a large Dutch oven over medium-high heat. Coat pan with cooking spray. Add brisket to pan; cook 10 minutes, browning on all sides. Remove brisket from pan. Add onion and oregano to pan; sauté 3 minutes. Return brisket to pan; add 1/2 cup water. Cover; reduce heat, and simmer 2 hours. Add olives and tomatoes; cover and cook 1 hour. Remove brisket from pan. Let stand 5 minutes. Cut brisket against grain into thin slices; return brisket slices to pan. Cover and cook over medium-low heat 30 minutes.