Sprinkle the brisket evenly with ¼ teaspoon salt and ¼ 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 ½ 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 to pan.
Cover and cook over medium-low heat 30 minutes.