Preheat your oven to 375 °F. Line baking sheets with parchment paper and set aside.
Sift together the flour, baking soda, salt, cinnamon and nutmeg in a small bowl and set aside.
In a medium sized mixing bowl, cream together the butter and sugars until the mixture is light and fluffy. Beat in the egg until fully incorporated, then add in the vanilla.
Mix in the banana, oatmeal and chocolate chips until fully combined.
Slowly add the dry ingredients into the wet, mixing in completely.
Drop by tablespoonful onto the prepared baking sheets.
Bake in the oven for 13-15 minutes until the edges are light brown and set. If you are finding that they are browning too quickly, reduce your oven temperature to 350 °F.
Remove from the oven and cool for 5 minutes on the baking sheets, then remove to fully cool on wire baking rack.
Store in a closed container at room temperature for 3-4 days. These can also be frozen.
Notes
If your cookies are browning too rapidly at 375 °. turn your oven down. Banana adds a lot of sugars to the cookie and the fruit sugars can cause cookies to burn easier! It all depends on how sweet your banana are!