Preheat oven to 350°.
In a large bowl, cream the butter and brown sugar together for 2-3 minutes. Add in the baking soda and flour and mix just until well combined.
Mix in the oats. The mixture will be a little bit crumbly but the oats should mix in well. Take 2 cups of the cookie dough out of the bowl and save for later. Press the remaining cookie dough into the bottom of a 10X15X1 cookie sheet. Bake for 15-18 minutes, or just until the cookie layer begins to turn a light golden brown.
Remove from the oven and spread the pie filling over the crust. Stir the cinnamon into the remaining cookie mixture and then sprinkle over the top. Lightly press the topping into the pie filling. Bake another 20-25 minutes or until the top turns a light golden brown.
Cool completely and slice into bars. Makes about 40 bars.