Preheat oven to 350°F and line baking sheets with parchment paper
Whisk together flour, oats, cinnamon, baking soda, and salt
Cream butter and brown sugar until combined, then mix in egg and vanilla
Toss chopped apples with lemon juice separately
Mix dry ingredients into wet ingredients until just combined
Fold in apples gently
Scoop dough into 2 tablespoon portions and space 2 inches apart
Flatten cookies slightly before baking
Bake for 14 to 15 minutes until edges are set
Cool on baking sheet before transferring to a wire rack