Preheat oven to 350°.
In a large bowl, combine flour, oats, brown sugar and salt. Using a pastry blender or fork, cut in butter until mixture is crumbly. Reserve 1/4 of crumb mixture for topping. Press remaining crumb mixture into bottom of ungreased 9″x 13″ baking pan. Bake 10 minutes.
In a small saucepan, heat dulce de leche over low heat for 3-4 minutes, stirring frequently, to soften slightly.
Spread dulce de leche over crust. Sprinkle evenly with toffee bits. Then, sprinkle with remaining crumb mixture.
Bake 25-30 minutes or until golden brown. Cool 15 minutes.
Run a knife around sides of pan to loosen bars. Allow to cool completely before cutting into bars.