Place the olive oil, lemon juice, oregano, salt and pepper in an empty clean glass jar put the lid on and shake well until everything is blended together. Put it to one side and begin making the salad.
Cut the tomatoes in quarters lengthways. Then cut the red onion into thin slices. Cut the green pepper into thin ribbons. Slice the cucumber thinly. Put everything into a salad bowl and add the olives and crumbled feta cheese. Shake up the salad dressing you made earlier and drizzle over the other ingredients. Mix the salad to ensure everything is coated evenly.