Cook onion, garlic, oregano, and red pepper flakes in 1 tablespoon oil in a 4-quart heavy pot over moderately high heat, stirring, until onion is softened, about 3 minutes.
Add wine and boil until reduced by 1/2, about 3 minutes. Stir in tomatoes and salt, then reduce heat, and simmer briskly, stirring frequently, until slightly thickened, about 8 minutes. Stir shrimp into sauce and simmer, stirring occasionally, until shrimp are just cooked through, about 3 minutes.
While sauce and shrimp are cooking, cook orzo in a 6-quart pot of boiling salted water until al dente. Reserve 1/2 cup cooking water, then drain orzo in a sieve. Return orzo to pot and toss with remaining tablespoon oil. Stir in sauce with shrimp and reserved cooking water, then add olives and salt and pepper, to taste.
Spoon 1/2 of pasta into an oiled 13 by 9 by 2-inch glass baking dish, then sprinkle with 1/2 of feta. Top with remaining pasta and feta, then bake in middle of oven, uncovered, until cheese is slightly melted and pasta is heated through, 10 to 15 minutes.
