In a medium bowl, mix together the oats, flour, baking soda, salt, cinnamon, nutmeg, and all spice (if using). Set aside.
In a large mixing bowl add the softened butter, brown sugar, and granulated sugar. Using an electric mixer, cream butter and sugar until well combined.
Add the egg and vanilla to the mixing the bowl. Continue to mix until well combined.
Slowly add the flour mixture on low speed and mix until just combined and no more flour streaks. Stir in the diced apples with a spoon or spatula and mix until well incorporated.
Cover and chill in the refrigerator for 30 minutes.
Preheat the oven to 350°F. Remove dough from the refrigerator. Prepare two baking pans with parchment paper.
Scoop dough using a medium cookie scoop, or ~2 tablespoons at a time. Roll into a ball and drop onto the prepared baking pans. Gently press down with your hand to flatten each ball of cookie dough slightly. If the apples start to come out, just gently press them back in.
Bake in the oven for 14-15 minutes until top is lightly golden. Remove from the oven and let cool on baking pan for five minutes. Then transfer to a wire rack to cool completely.
Once cooled completely, store the cookies in an airtight container.