Combine the tomato sauce, canned tomatoes, minced onion, salt, pepper and oregano in a saucepan. Cover and simmer for 20 minutes.
Bring a large pot of lightly salted water to a boil. Add pasta shells and cook for 8 to 10 minutes or until al dente. Drain and rinse with cool water.
Put a layer of cooked shells in a greased 3 quart casserole dish. After the shells, you put in the cottage cheese and spread it out to cover all the shells. Layer on the sauce, then another layer of shells, then the mozzarella cheese and more sauce. Add the last layer of shells, the parmesan, and finish with a layer of sauce.
