As Grims notes, they're not filled with butter, but they have lots of sugar (which you can cut back on, particularly in the fruity variety).
Preheat the oven to 375.
Spray a 9/13 baking dish (glass and metal both work fine) with non-stick spray.
Mix the following in a large bowl with a big spoon:
1/2 cup milk (or a bit less, if you're adding fruit)
1 1/2 cups brown sugar
1 tsp. vanilla
1 1/2 cups flour
1 tsp. baking soda
1 tsp. baking powder
1/2 tsp. salt
4 cups oats (I prefer the old fashioned variety to the quick oats)
1/2 bag chocolate chips*
Press into baking dish and bake for 15-20 minutes; they should be gooey.
You can also add 1/2 to 1 cup peanut butter to the recipe above (yummy).
Or you can experiment with different fruits. I've added some peeled chopped apples (with some cinnamon and nutmeg to taste) and peeled, chopped peaches. Both are excellent. But lots of fruit would work. Just add less milk if you use fresh fruit, or otherwise you'll have to add more flour and they'll be cakey rather than chewy, and they're much better chewy.
Let cool for about 10 minutes, particularly with the fruity ones. Cut into the size you want and enjoy.