Put the oats, sugar and salt in a food processor and process to a fine meal. Add the flour and process to mix.
Scatter the butter pieces over the dry ingredients. Pulse until the butter is broken into very small pieces, about the size of a pea.
In a small bowl, stir the yolk and 2 tablespoons of the water. Sprinkle over the flour. Pulse several times. Mix with a fork to insure that the ingredients are mixed well. Pulse again until the pastry just starts to gather in a mass. Dump into a bowl and gather everything together.
Form into a disc about ¾-inch thick. Wrap in plastic wrap and refrigerate at least one hour or overnight.
Cream Cheese Pie Dough:
Beat the butter in the bowl of an electric mixer until it is smooth. Add the cream cheese, sugar and salt. Beat well, stopping to scrape the sides and the bottom of the bowl. Add the flour all at once, and mix just until the dough comes together.
Gather the dough and form into a disk, about ¾-inch thick. Wrap in plastic wrap and refrigerate at least one hour or overnight.
Notes
If you use either pie dough for a savory tart, leave out the sugar.