How to Make It
In a large bowl, whisk the olive oil with the lemon juice, garlic and oregano. Season the dressing with salt and pepper.
In a large pot of boiling salted water, cook the green beans until tender, about 4 minutes. Using a slotted spoon, transfer the beans to a colander. Add the potatoes to the pot and cook until tender, about 10 minutes. Using a slotted spoon, transfer the potatoes to the colander. Add the orzo to the boiling water and cook until al dente; drain well.
Add the green beans, potatoes and orzo to the dressing and toss gently to coat. Fold in the tuna, cherry tomatoes and fresh herbs and season with salt and pepper. Serve warm or at room temperature.