Prep the produce: chop the tomatoes, slice the onion, and mince the garlic.
Whisk the olive oil, red wine vinegar, salt, pepper, oregano, and sugar in a large bowl.
Add tomatoes, onion, and garlic to the bowl and toss until evenly coated.
Let the salad rest 5–10 minutes for flavors to meld.
Fold in the basil, taste, and adjust seasoning before serving.