In large mixing bowl, cream butter, shortening and sugars. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine flour, baking soda and salt; gradually add to creamed mixture. Stir in oats, chocolate chips and marshmallows.
Drop by heaping teaspoonsful 2-inches apart onto greased baking sheets. Bake at 350° 11 to 13 minutes or until golden brown. Cool 1 to 2 minutes before removing pans to wire racks to cool completely. Makes about 6 dozen.