To make the dressing, combine the vinegar, garlic, mustard, oregano, and salt in a smallish bowl. Whisk until thoroughly combined, then continue whisking as you drizzle in the olive oil. When all the olive oil is incorporated, set the dressing aside. (Or you can put all the ingredients in a small jar with a tight-fitting lid and just shake it emphatically.)
Combine the salad greens, tomatoes, cucumber, bell pepper, onion, and 1/2 cup of the feta in a large bowl, and toss to mix well.
Just before serving, whisk the dressing - or shake it, if it's in a jar - to recombine, and add about half of it to the salad. Toss to coat, and give it a taste. You might want to add the rest of the dressing right now, or bring it to the table (along with the pepper mill) for people to add more to their own portions. Top with the remaining 1/2 cup feta, the olives, and a few grinds of black pepper, and serve immediately.