Seared Scallops with Orange and Red Onion Salad

1/2 red onion, thinly sliced 

1 1/2 Tbs. rice vinegar 

3 oranges 

1/2 cup pitted mild green olives 

2 Tbs. extra-virgin olive oil 

1 lb. sea scallops, each 1 1/2 to 2 inches in diameter 

Salt and freshly ground pepper, to taste 

2 Tbs. coarsely chopped fresh mint