Preheat the oven to 180ºC and line a baking sheet with baking paper.
If you don't have oat flour, grind the whole oats into a flour using a food processor.
Add the oats, banana, peanut butter, flax, cinnamon and salt to the food processor, and blend until smooth.
Add a bit of water to help the dough form – but it should still be a thick, sticky dough.
Take a heaped teaspoon of dough into your hand and roll it into a ball, then place on the baking sheet.
Repeat until the dough is used up, then take a fork and press each ball into a flattened cookie shape.
Bake for 20 minutes, then leave on a cooling rack until at room temperature.
Store in an airtight container at room temperature for up to a week, or freeze for up to 3 months.