Preheat broiler. Arrange red onion slices on a 15x10x1-inch baking pan. Broil 4 to 5 inches from the heat for 8 to 10 minutes or until charred and tender, turning once halfway through broiling time. When cool enough to handle, separate onion into rings, discarding any large dry blackened pieces.
In a medium bowl combine onion, grapefruit juice, orange juice, lime juice, salt, oregano, garlic, and pepper. Cover and chill for at least 2 hours before serving.