Preheat the oven to 350 degrees and lightly grease a loaf pan.
In a large mixing bowl, combine the ground beef, rolled oats, chopped onion, eggs, milk, dried thyme, salt, and black pepper. Mix until evenly blended but do not overwork the meat.
Transfer the mixture to the prepared loaf pan and press it into an even shape.
In a small bowl, stir together the ketchup, brown sugar, and yellow mustard until smooth. Spread this mixture evenly over the top of the meatloaf.
Bake for 55 to 60 minutes until the internal temperature reaches 160 degrees. Let the meatloaf rest for 10 minutes before slicing.