To prepare dressing: Place orange zest, orange juice, vinegar, oil, oregano, mustard, salt and pepper in a jar. Cover and shake to combine.
To prepare salad: Slice both ends off orange. With a sharp knife, remove the peel and white pith; discard. Working over a large bowl, cut the segments from their surrounding membrane. Squeeze the juice into the bowl before discarding the membrane, if desired. Add lettuce, radishes, beans, scallion, feta and dressing; toss to coat. (Refrigerate the extra dressing for up to 1 week.)