Combine the vinegar, lemon juice, oregano, and salt and pepper to taste in a small bowl and whisk together. Add the oil in a steady stream, whisking constantly.
Combine the lettuce, onion, bell pepper, tomato, cucumber, olives and feta in a large salad bowl. Pour the dressing over and toss until all of the ingredients are well coated. Taste and adjust the seasonings, adding more salt or lemon juice as desired.