Heat oven to 350°F. In large bowl, mix flour, oats, brown sugar and salt. With pastry blender or fork, cut in butter until mixture is crumbly. Press 3/4 of mixture in ungreased 13x9-inch pan.
Bake 10 minutes. Meanwhile, in 1-quart saucepan, heat dulce de leche over low heat 2 to 4 minutes, stirring frequently, until slightly softened.
Spread dulce de leche over partially baked crust. Sprinkle evenly with toffee bits and remaining crumb mixture. Bake 20 to 25 minutes or until golden brown. Cool 15 minutes.
Run knife around sides of pan to loosen bars. Cool completely, about 30 minutes. For bars, cut into 8 rows by 4 rows.