Bring large pot of water to boil. Cut onion into thin slices. In a large skillet, warm the oil over medium-high heat until hot but not smoking. Add the onion and garlic and cook until the mixture is golden, about 5 minutes. Add the tomatoes, oregano, basil and pepper, and bring the mixture to a boil over medium heat, breaking up the tomatoes with a spoon. Reduce the heat to low, cover and simmer, stirring occasionally, while you cook the pasta and prepare the shrimp. Shell and devein the shrimp. Uncover the sauce, bring to boil over medium-heat. Add the shrimp, reduce heat to low, cover andsimmer for 3 minutes. Add the olives and half the feta cheese, stir to combine. Drain the pasta, serve topped with the shrimp and sauce. Crumble remaining feta cheese on top.