Preheat the oven to 350 F. Mix the oatmeal, brown sugar, flour, and salt in a bowl. Stir in the melted butter, then add the egg and vanilla and combine well. Arrange the batter by half-teaspoonfuls, about 2 inches apart, on ungreased cookie sheets. Bake until lightly browned, about 5 minutes. Cool slightly, removing the cookies from the cookie sheet with a spatula as soon as they are firm. If they become too hard, return to the oven for a few minutes to soften.