Cream together sugar and butter; stir in egg; combine flour, baking powder, cinnamon, and salt; add to butter mixture; mix to form a soft dough; reserve a half cup dough to be used later for topping; refrigerate the remaining dough.
In a medium-sized bowl, whip egg whites until stiff; combine egg yolks, sugar, ricotta cheese, sour cream, flour, salt, orange juice, and vanilla in a large bowl and beat until smooth; gently fold in whipped egg whites.
Press chilled dough evenly into a 10 springform pan that has been lightly sprayed with no-stick cooking spray; pour filling over crust; combine reserved dough with oatmeal and cinnamon; crumble this topping evenly over filling.
Bake at 350° for 45-60 minutes or until a knife inserted near center of cheesecake comes out clean.
Cool cheesecake on rack; cover and refrigerate to chill thoroughly; just before serving remove sides of springform pan and transfer cheesecake to serving platter.