Make the marinade. In a small bowl, whisk together the olive oil, balsamic vinegar, dried oregano, dried basil, salt, and pepper until well combined. Set aside.
Prep the cucumber. Peel the cucumber, then run a fork along the outside to score it. Slice thinly, then dice into ½-inch pieces. Add to a medium mixing bowl.
Add tomatoes and onion. Quarter the cherry tomatoes and add them to the bowl. Finely chop the red onion and add it as well.
Add the mozzarella. Halve the mozzarella balls (or tear a larger ball into bite-sized pieces) and add to the bowl. Gently mix to distribute evenly.
Dress and marinate. Pour the marinade over the salad ingredients and toss gently until everything is evenly coated. Let the salad rest for 10 minutes before serving to allow the flavors to meld.
Garnish and serve. Top with fresh basil leaves if using, and serve immediately.