Halve the cherry tomatoes and slice the cucumber into thin half-moons. Toss both with ½ teaspoon salt in a colander set over a bowl or the sink. Let drain for exactly 10 minutes.
While the vegetables drain, chop the romaine into 1-inch ribbons. Wash and dry thoroughly, then add to a large salad bowl.
In a small jar or bowl, combine the olive oil, red wine vinegar, dried oregano, a pinch of salt, and a few cracks of pepper. Shake or whisk until emulsified.
Give the drained cucumber and tomato a final shake to remove any remaining liquid, then add them to the bowl with the lettuce, along with the sliced red onion.
Pour the dressing over the salad and toss gently but thoroughly, adding the feta during this step if using.
Serve immediately for the crispest texture.