why does my meatloaf fall apart when I slice it? My recipe:

1.5 lbs of 90/10 lean ground beef
.75 c oatmeal
.75 c chopped onion
1 extra large beaten egg
1 tbsp worchestershire sauce
3-4 chopped garlic cloves 
.5 tsp salt
.5 tsp black pepper
I bake at 350 F until thermometer reads 170 F and let it rest in pan for 10 minutes before plating and slicing but even with a super sharp knife it falls apart.
It's the oatmeal.  I suspect it is absorbing the egg and preventing it from binding the meat together.  I suggest switching to a small amount of breadcrumbs, or day old bread torn into very small pieces if you like a little more texture.