Preheat oven to 350 degrees. Line a baking sheet with parchment paper.
In a large bowl, stir together the flour, salt, baking soda and oats. Set aside.
Melt the butter on the stove in a small pan; do not allow it to brown. Or place it in a glass cup and melt in a microwave oven set on high for 50 seconds. Pour the margarine into a medium-size mixing bowl and add the sugars, banana and vanilla extract. Using a rubber spatula or a wooden spoon, stir until smooth.
Add the butter mixture to the flour ingredients and stir until it forms a dough. Stir in the chocolate chips
Spoon the dough by level tablespoons onto the prepared baking sheet, spacing them about 1 inch apart. Bake the cookies on the middle rack of the oven until lightly puffed and golden, 8 to 10 minutes. They should be a little soft, but not too squishy in the center.
Remove from the oven and transfer to wire racks. They will firm up further as they cool. Serve the cookies warm or cool.