To make the crust, beat butter with electric mixer until creamy. Add confectioners' sugar; beat well. Then,add combined flour and oats; beat on low speed until crumbly.
Press crumbs firmly and evenly onto bottom of Greased 9 x 13 pan. Bake 20 minutes, until light golden brown. Remove from oven. Reduce heat to 300ºF.
While the crust is baking, prepare the filling. Beat eggs, granulated sugar and orange zest until well blended. Add juice; beat to combine. Add flour and mix just enough to combine. Pour filling over warm, baked crust.
Bake the crust with filling until filling is set, about 40 minutes.
Cool in pan 30 minutes. Cover and refrigerate.
To serve, cut into bars. You may choose to garnish with more orange zest or confectioner's sugar.