Combine oats, boiling water and butter in a large bowl; let stand for 5 minutes. Dissolve yeast from roll mix in warm water; add to oat mixture. Stir well; mix in eggs.
Add flour from roll mix; blend well. Form into a 10-inch circle on a greased 12" pizza pan. Cover; let rise for 25 minutes, until double.
Bake at 350 degrees for 25 minutes. Cool on a wire rack; cut in half horizontally. Whisk together mayo and mustard; spread over cut sides. Assemble sandwich and cut into wedges.