Heat the olive oil in a large frying pan over medium-high heat.
Add the sweet Italian sausages to the pan and cook until browned all over, about 8–10 minutes. Transfer to a plate and set aside.
Reduce the heat to medium and add the sliced onion and peppers. Cook, stirring occasionally, until softened but still slightly crisp, about 8–10 minutes.
Stir in the minced garlic and cook for about 1 minute, until fragrant.
Add the chopped tomatoes, then stir in the dried oregano, salt and black pepper.
Return the sausages to the pan, nestling them into the tomato mixture, and bring to a gentle simmer.
Cover the pan, reduce the heat to low and cook for 20–25 minutes, or until the sausages are cooked through and the flavours have melded.
Taste and adjust the seasoning if necessary, then garnish with fresh basil leaves before serving.