Cover olives with cool water, and let stand 30 minutes.
Heat grill to medium-high. Brush tomatoes and red onion with 1/4 cup olive oil. Grill until slightly charred and softened. Immediately chop tomatoes and onion, and toss with garlic while still hot. Season with salt and pepper; cool to room temperature.
Mix together vinegar and anchovy paste. Slowly drizzle in remaining 1/4 cup olive oil, whisking constantly, until slightly thickened. Drain and chop olives. Combine chopped tomatoes and onion with olives, capers, and oregano. Drizzle vinaigrette over vegetables, and toss gently to coat.