Generously grease a 13x9 pan or coat with nonstick spray.
For the filling: in a medium heavy nonreactive saucepan, stir together the sugar and cornstarch till well blended. Stir in the rhubarb, jam or preserves, cinnamon and orange zest, is using. Bring to a simmer, stirring over medium high heat. Cook, stirring, for 3-6 minutes or until rhubarb softens just slightly, fresh rhubarb will take longer than thawed frozen. Set aside. For the crumb mixture; In a large bowl, thoroughly stir together the flour, oats, sugar, cinnamon and salt. Add the butter and stir or knead with your hands till the mixture is well blended and crumbly. Firmly press a scant half of the crumb mixture into the baking dish; set aside the remaining crumb mixture. Bake in the middle of the oven for 13-16 minutes or until firm but not browned. Spread the rhubarb filling evenly over the crumb layer.
Stir together the reserved crumb mixture and 2 1/2 tbls of the egg mixture until the streusel forms small clumps; add a little more of the egg mixture if needed.
Sprinkle the streusel evenly over the filling, breaking up any large clumps with a fork or your fingertips.
Bake in the middle of the oven for 35-40 minutes, or until the streusel is nicely browned and the filling is bubbly.
Transfer the pan to a wire rack and let stand until completely cooled.
Using a large sharp knife, cut into 20 bars -- wipe the knife clean between cuts.
Store in an airtight container for up to 2 days. The bars may be frozen for up to 1 month, but will lose their crispness.