Heat oil in a large nonstick skillet over medium-high heat. Add onions and garlic, and sauté 2 minutes. Add tomato, and cook over medium heat 7 minutes. Add tomato paste, stirring well to combine. Add capers and salt; cook 15 minutes or until mixture begins to thicken. Stir in shrimp and oregano; cook 3 minutes or until shrimp are done. Combine shrimp mixture and pasta in a large bowl, tossing gently. Sprinkle with cheese and pepper.