Place oats in the bowl of a food processor; pulse until finely ground.
Transfer ground oats to a medium bowl; stir in flour, sugar, and salt. Cut in butter with a pastry blender or two knives until mixture is crumbly. Sprinkle with 2 tablespoons water. Blend with a fork until mixture holds together, adding up to 1 tablespoon more water if necessary.
With minimal shaping, form dough into a ball and place on a lightly floured sheet of waxed paper. Place another sheet of waxed paper on top and roll dough into a 1/8-inch thick circle large enough to fit a 9-inch deep-dish pie plate. (This dough breaks more easily than a white flour dough; the wax paper helps with that.)
Remove top sheet of waxed paper and invert dough into the pie plate. Slowly peel away remaining sheet of waxed paper and trim any excess crust. Turn dough edge under and crimp as desired.
Crust is ready to fill. See notes if your recipe requires a pre-baked crust.