Saute the onions, garlic and green pepper in the oil until light brown, stirring often. Add the parsley, salt, pepper and oregano and cook, stirring, until the parsley is wilted but not brown.
Add the sour cream and mix. Add the ricotta and heat to serving temperature. To prevent curdling, do not heat to simmering.
Meanwhile, cook pasta until al dente. Drain well, return to the pot and add the sauce. Toss until well mixed. Adjust the seasonings and serve on a hot platter.