Zest 2 of the 3 oranges and put the orange zest in a large, shallow dish. Juice half an orange and add to the zest.
To the zest and juice, add the garlic, 3 Tbs EVOO, 1/4 t. cayenne, salt, and pepper, and stir to combine.
Add the chicken breasts, toss to coat thoroughly, and let marinate for 5 minutes.
While the chicken is marinating, with a paring knife cut the skin and white pith from the 2 remaining whole oranges, being sure to remove it all. Slice the oranges into disks and place in a salad bowl.
Cut the radicchio in quarters through the core, cut the core off from each quarter, and then thinly shred the radicchio and add to the oranges.
Add the red onions and arugula.
To make the dressing, juice the other half orange into a small bowl and add the pinch of cayenne, the oregano, honey, and mustard. In a slow, steady stream, whisk in the 4 remaining Tbs of EVOO.
Transfer the marinated chicken to the grill and cook on each side for 5 to 6 minutes, or until cooked through.
Remove from the grill and let rest for a few minutes. Lightly toast the bread.
Thinly slice the grilled chicken on an angle, add to the salad, and then pour the dressing over. Toss to combine and serve with the toasted bread.