Bring large pot of water to a boil. Add green beans; boil until crisp-tender, about 6 minutes. Remove the green beans with a slotted spoon to a colander; immediately rinse with cold running water to stop the cooking. Lightly salt the boiling water. Add the pasta to the boiling water; cook pasta following package directions. Drain in a colander, rinsing with cold water; let the pasta cool.
In a large bowl, combine green beans, the cooled pasta, cucumber, red onion, tomatoes, tuna and olives until well mixed.
In small bowl, whisk together the mustard, lemon juice, vinegar, sugar, salt, oregano and black pepper. While whisking, add the oil in a thin stream; continue to whisk the dressing until smooth and the oil is thoroughly incorporated.
Pour dressing over the top of the salad; gently toss the salad with 2 large spoons to evenly coat all the ingredients with the dressing. Serve. Makes 12 servings.