1. Preheat oven to 350ºF. Line two large baking sheets with parchment. Combine flour, baking soda and salt in a bowl.
2. In a large bowl, using an electric mixer on medium speed, beat together butter and brown sugar until well combined. Beat in egg and vanilla. Stir in flour mixture until just combined. Fold in oats and gumdrops.
3. Drop dough by heaping tablespoonfuls onto prepared baking sheets, leaving 2 inches between each cookie. Bake until golden around edges but still soft on top, about 15 minutes. Let stand on baking sheets for about 5 minutes, then transfer to wire rack and cool completely.