For the Crust:
Whisk together flour, oats, brown sugar, baking soda and salt in a large bowl.
Using a pastry cutter or two knives, cut in butter until mixture resembles coarse crumbs.
Press half of the mixture into prepared pan, reserving the remaining half, and bake for 10 minutes.
For the Filling:
Meanwhile, combine sugar and water in a medium saucepan over medium heat, stirring until sugar dissolves. Increase heat to medium high and bring to a boil.
Boil, without stirring, until mixture becomes deep amber in colour, 5 to 8 minutes. Remove from heat and stir in cream and butter. The mixture will bubble up vigorously.
Return to low heat and stir until caramel becomes smooth, 1 or 2 more minutes.
Pour caramel over prepared crust and sprinkle with chocolate chips and nuts, if using. Sprinkle with remaining crumb mixture and bake 15 to 20 minutes, or until caramel is bubbling and top is light golden.
Remove from oven and cool completely. Cut into bars.