Cook spaghetti to desired doneness as directed on package.
Meanwhile, heat oil in large nonstick saucepan over medium-high heat until hot. Add onion and garlic; cook and stir 4 minutes.
Stir in tomatoes, reserved clam liquid, Italian seasoning and salt. Reduce heat; simmer 15 minutes.
Drain spaghetti. Stir clams and olives into sauce. Serve sauce over spaghetti.