Preheat your oven to 350F/175°C. Chill a large mixing bowl in the fridge.
Line the bottom of an 8-inch pie dish with parchment paper.
In a blender or food processor, blend the oats on high until they form a smooth flour (about 30 seconds). Keep blending until the flour is as fine as possible.
To your large mixing bowl, add the oat flour, sea salt and arrowroot. Mix with a metal spoon to combine ingredients.
Remove the hardened coconut oil from the fridge and using your hands, quickly rub it into the flour mixture until it resembles fine breadcrumbs (we're doing this quickly so that the oil melts as little as possible).
Add the maple syrup and water to the bowl and mix these into the pastry using a metal spoon.
Use your hands to quickly shape the mixture into a ball of dough.
Place a sheet of parchment paper on a flat surface, and place the dough on top. Place another sheet of parchment paper on top of the dough.
Using a rolling pin, roll your dough out until it is about an inch wider than the circumference of your pie dish and roughly 6mm thick.
Remove the top layer of parchment paper from your pastry and place the pie dish upside down on top of it. Gently but quickly flip the whole thing over so your pastry is on top of the dish! (Be careful during this part, as the pastry can be crumbly. If it breaks, don't worry - it should be fixable in the next step!)
Remove the remaining layer of parchment paper, and very gently press the pastry into the pie dish, ensuring it reaches the corners. If you get any cracks at this point, simply use a piece of pastry from the edge to plug them.
Using a flat knife, cut the edge of the pastry around the rim of the dish so you have a nice clean crust. (You will likely end up with a little spare pastry - feel free to use this as you wish! I like making jam tarts or cookies from it.)
Bake in the oven for 15-20 minutes, until the crust is just starting to turn golden.
Remove and allow the crust to 'rest' in its pie dish for 10 minutes, then turn out onto a wire rack to cool completely. (Do not remove the crust from the dish before this! It will be soft and will likely break.)