Preheat the oven to 375°F and lightly grease a loaf pan. In a large mixing bowl, combine the ground beef, rolled oats, gochujang, soy sauce, egg, onion, and garlic. Mix with your hands until the oats and seasonings are evenly distributed.
Season the mixture with salt and pepper, then press it firmly into the prepared loaf pan. Bake for 50 to 55 minutes until the internal temperature reaches 160°F.
While the meatloaf bakes, place the sliced cucumbers in a medium bowl. Add the sesame oil, rice vinegar, and sesame seeds. Toss gently and season with a pinch of salt. Let the slaw sit at room temperature to develop flavor.
Remove the meatloaf from the oven and let it rest for 5 minutes. Slice and serve topped with the sesame cucumber slaw.