HEAT oven to 350ºF.
HEAT oil in medium skillet on medium heat. Add onions; cook and stir 5 to 6 min. or until crisp-tender. Add garlic; cook and stir 1 min. Remove from heat.
BEAT 1 egg in large bowl. Add meat, oats, dressing, parsley, oregano, pepper and onion mixture; mix just until blended.
SHAPE 2/3 of the meat mixture into shallow loaf in 13x9-inch baking dish sprayed with cooking spray. Press down center of loaf to make indentation.
BEAT remaining egg in medium bowl. Add spinach and feta; mix well. Spoon into indentation in meatloaf; cover with remaining meat mixture. Press edges together to seal, completely enclosing spinach mixture. Top with mozzarella; cover.
BAKE 1 hour or until meatloaf is done (160°F), uncovering for the last 10 min. Let stand 10 min. before slicing to serve.