Combine flour, baking soda, and salt in a bowl and lightly whisk to combine.  Set aside.

In a mixing bowl or stand mixer, melted butter, brown sugar, and white sugar.  Add eggs, one at a time, stirring between each addition, and vanilla.  Add flour mixture in a couple additions until incorporated.

Stir in oats and chocolate chips until just combined.  *I like to just sprinkle my M&M's on the top of the dough, but if you have a big bag, feel free to toss a small handful into the dough, or substitute M&M's for a portion of your chocolate chips.

Press dough into prepared pan and press flat.  Sprinkle a handful of M&M's on top and add sprinkles if you desire.

Bake for about 25-30 minutes. Remove from oven when edges are golden brown and center appears set, but still soft. The center should jiggle a bit when you move the pan.  They will be very doughy in the middle!  Unless you are serving slightly warm over ice cream, cool completely to room temperature.  Note: It's totally normal for the center to sink down lower than the edges, once cooled! I think these are best when they've cooled for several hours and I like to put them in the fridge to chill before cutting, even.  They are great to make ahead!