In extra large saute pan, melt butter and add diced onion. When onion gets warm, add 2 cups uncooked rice. Saute butter, onion and rice, until rice is all golden brown. Add soup, water, drained mushrooms, oregano leaves, salt and pepper. Bring to a boil, cover and reduce heat to low. Cook 15-20 minutes or until rice is done. Do not stir once covered. Stir right before serving.