Preheat the oven to 375 degrees F (190 degrees C). Line a baking sheet with parchment paper.

Mix oats, brown sugar, flour, and salt together in a bowl. Stir in melted butter until well blended. Pour in egg and vanilla extract and mix until well blended.

Drop dough by 1/2 teaspoonfuls, at least 2 inches apart, on the prepared baking sheet.

Bake in the preheated oven until lightly browned, 4 to 5 minutes.

Remove from the oven and let cool slightly until firm, 5 to 10 minutes. Transfer cookies to wire racks to cool completely. Continue baking remainder of cookies, 1 cookie sheet at a time, according to instructions.