In 3 1/2- to 4-quart slow cooker, place chicken thighs. Top with onion, olives and capers. Sprinkle with oregano, salt, rosemary and garlic powder. Pour tomatoes over chicken.
Cover; cook on Low heat setting 7 to 10 hours.
About 15 minutes before serving, with slotted spoon, remove chicken and vegetables from slow cooker; place on serving platter. Cover to keep warm.
In small bowl, blend water and cornstarch until smooth. Stir into liquid in slow cooker. Increase heat setting to High. Cover; cook 5 to 10 minutes longer or until thickened. Serve chicken with sauce.