Preheat oven to 350 degrees F and line 2 cookie sheets with parchment paper.
In a large bowl, cream butter and both sugars.
Beat in egg, then stir in vanilla.
In a second bowl, combine oats, flour, salt, baking soda, and cinnamon and stir into butter mixture.
Stir in the M&M's.
Place tablespoons of dough about 2 inches apart on cookie sheets. Flatten each cookie slightly with a large fork.
Bake 10 to 12 minutes, or until lightly browned around the edges. Cool for 5 minutes before transferring to a wire cooling rack.