Cook the orzo in a large pot of salted water until al dente, according to package instructions.
Drain, then transfer to a large mixing bowl.
While the orzo cooks, make the dressing: In a bowl or liquid measuring cup with a spout, whisk together the oil, vinegar, garlic, mustard, salt, oregano, and black pepper.
While the orzo is still warm, pour about half of the dressing over it and stir to coat.
To the bowl with the orzo, add the cucumber, tomatoes, bell pepper, chickpeas, feta, olives, and basil.
Toss to combine, adding more dressing so that everything is nicely moistened.
Taste and add more salt and pepper for additional seasoning or a splash or two of vinegar for more zing.
Refrigerate until ready to serve; just before serving, toss with a bit more dressing.