In a small jar, combine olive oil, fresh lemon juice, ground oregano and a dash of salt. Shake well until the dressing is evenly mixed.
In a large bowl, add the spinach, cucumber, avocado, tomato, feta cheese, radish, and red onion. Pour the lemon dressing over the salad and toss until all ingredients are well-coated. Finally, add croutons on top and enjoy.