Preheat oven to 350 degrees.
Cream the butter and sugar together.
Add the egg and mix well.
Combine the flour, baking soda, and cinnamon.
Add the dry ingredients to the creamed mixture alternately with the mashed banana.
Stir in the rolled oats, cocoa chips, and chopped nuts, if desired.
Drop by teaspoonfuls onto a lightly greased cookie sheet.
Bake for 12 minutes.