Cook pasta according to package directions. Drain; keep warm. Meanwhile, drain canned clams, reserving the juice from one of the cans (you should have about 1/2 cup). Add enough of the half-and-half to the reserved clam juice to equal 2 1/2 cups liquid. Set clams and clam juice mixture aside.
In a medium saucepan cook onion and garlic in hot butter over medium heat until tender but not brown. Stir in flour, dried oregano (if using), salt, and pepper. Add clam juice mixture all at once. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Stir in drained clams, fresh oregano (if using), parsley, and wine or broth. Heat through. Serve over hot pasta. Sprinkle with Parmesan cheese. Makes 4 servings.