In a large skillet over medium heat, heat oil. Cook onion until soft, about 5 minutes.
Stir in tomato paste and garlic and cook until fragrant, about 1 minute more.
Add ground beef and cook, breaking up meat with a wooden spoon, until no longer pink
(approx. 6 minutes). Drain fat.
Return beef mixture to skillet, then stir in cooked rice and diced tomatoes.
Season with oregano, salt, and pepper. Let simmer until liquid has reduced slightly, about 5 minutes.
Place capsicums cut side-up in a 9"-x-13" baking dish and drizzle with oil.
Spoon beef mixture into each pepper and top with cheddar cheese, then cover baking dish with foil.