Prep: 20 minutes; Cook: 7 minutes.
1. Slice off top and bottom of each orange with a serrated fruit knife or sharp paring knife, removing some flesh with the peel, and reserve. With flat end of orange on a cutting board, cut off peel with a sawing motion from top to bottom, working all the way around oranges. Working over a bowl to collect juice, cut between membranes to separate orange segments, and set aside.
2. Squeeze juice from orange tops, bottoms, and membranes into bowl (you should have about 1/2 cup), and strain into a sauté pan. Add vinegar, and bring to a boil over medium-high heat. Reduce heat to low, and simmer until reduced to about 1/4 cup, about 7 minutes. Pour hot liquid into a bowl, and whisk in olive oil in a slow, steady stream. Stir in salt and pepper.
3. Toss arugula with fennel, onion, and 1/4 cup of dressing. Divide among 4 plates, and divide reserved orange segments among plates. Drizzle with additional dressing, and top with olives. Serve immediately.