You may use any combination of tomatoes to make this salad; cut cherry tomatoes in half. Snip basil with sharp kitchen scissors rather than cutting it with a knife to prevent bruising.
Combine tomatoes and onions in a large serving bowl. Drizzle with the olive oil, and sprinkle with the salt and pepper. Cover with plastic wrap; let stand at room temperature at least 1 hour to allow flavors to develop. Snip basil, and toss into salad just before serving.