Cut the onion in thin slices and place in a medium bowl.
Cut the tomatoes in slices as well and add to the onions.
Put the olive oil, vinegar, oregano, garlic, salt, and pepper in a small glass bowl and whisk together. Pour over the tomatoes and onions. Stir together in a circular motion so all the veggies get covered with the dressing.
Serve right away.
This salad is best eaten right away, but you can certainly store any leftovers if you don't mind eating tomatoes that are softer.