In a large bowl, cream butter and brown sugar until light and fluffy. Beat in egg and vanilla. Combine the flour, baking soda and salt; add to the creamed mixture and mix well. Stir in oats.
Press into a greased 15x10x1-in. baking pan. Bake at 350° until golden brown, 10-15 minutes. Cool on a wire rack.
In a microwave, melt caramels with water; stir until smooth. Spread over crust. Sprinkle with the chips, nuts and M&M's. Gently press into the caramel mixture. Melt candy coating; drizzle over the top. Let stand until set, 5 minutes. Cut into bars.
Editor's Note: Confectionery coating, sometimes called "almond bark" or "candy coating", is found in the baking section of most grocery stores. It is often sold in bulk packages of 1 to 1-1/2 pounds.