In a medium bowl, whisk together the olive oil, vinegar, garlic, oregano, salt and pepper until combined.
Add the cucumber slices, avocado, feta and onion slices into the dressing and toss well. Taste test and adjust salt and pepper, if desired.Serve immediately.
Notes
Tip: If you like a little, spicy kick with your salads, omit the feta cheese and sprinkle with a small pinch of crushed chili flakes.