Heat oven to 350°F.
Spray 13x9-inch pan with cooking spray. In pan, break up cookie dough. With floured fingers, press dough evenly to form crust. Bake 16 to 19 minutes or until golden brown.
Sprinkle crust with marshmallows. Bake 2 to 3 minutes longer or until marshmallows are puffy.
Meanwhile, in small saucepan, melt chocolate chips with oil over low heat, stirring until smooth.
Sprinkle bars with candy pieces. Drizzle melted chocolate mixture over warm bars. Cool completely, about 45 minutes. For bars, cut into 6 rows by 6 rows.