In a small bowl, whisk together olive oil, red wine vinegar, oregano, sea salt and black pepper.
In a medium bowl, add in tomatoes and onions. Tomatoes should be sliced in wedges or cut in half or quarters if using cherry tomatoes. Onions should be thinly sliced.
Pour over olive oil vinegar mixture on the tomatoes and onions. Toss gently to coat well. Taste to check for seasonings and adjust if desired.
Serve garnished with extra herbs like chopped basil, fresh oregano or chives.
This tomato and onion salad is best served right away as tomatoes don't do well when refrigerated.