To make the meatballs, combine the beef mince, onion, breadcrumbs, egg, olives and oregano in a large bowl. Season with salt and pepper. Mix until well combined. Roll tablespoonfuls of mixture into balls. Use your thumb to push an indent into 1 meatball and place 1 bocconcini half in the indent. Push the mince around the bocconcini to enclose. Transfer to a plate. Repeat with remaining meatballs and bocconcini. Place in the fridge for 30 mins to chill.
Heat half the oil in a large non-stick frying pan over medium heat. Add half the meatballs. Cook, turning, for 5 mins or until brown all over. Transfer to a bowl. Repeat with remaining meatballs.
Add the remaining oil to the pan. Add the onion, garlic and chilli. Cook, stirring, for 5 mins or until onion softens. Add the tomato. Cook, stirring, for 5 mins or until sauce boils and thickens slightly. Return the meatballs to the pan with the chopped basil and oregano. Cook for 5 mins or until heated through.
Meanwhile, cook the spaghetti in a large saucepan of salted boilingwater following packet directions or until al dente. Drain well. Add to the tomato sauce and toss to combine.
Divide the spaghetti and meatballs among serving bowls. Serve sprinkled with extra basil leaves and parmesan.