In a medium bowl, whisk together flour, cinnamon, baking soda, and salt. Set the dry mixture aside while preparing the butter and sugar.
Beat the softened butter, brown sugar, and granulated sugar until creamy and slightly lighter in color. Add the egg and vanilla, then mix until smooth.
Add the dry ingredients to the butter mixture and stir just until no flour streaks remain. Fold in the oats, finely diced apple, and any optional additions.
Cover the dough and refrigerate it for 20 minutes. Meanwhile, preheat the oven to 350°F or 175°C and line two baking sheets with parchment.
Scoop rounded tablespoons of dough onto the trays, leaving about 2 inches between them. Gently flatten the tops because the chilled cookies will not spread dramatically.
Bake for 11 to 13 minutes, until the edges are lightly golden but the centers still look soft. Cool on the tray for 5 minutes before transferring to a rack.
