In a medium bowl, whisk together the oil, lemon juice, honey, shallot, lemon zest, garlic, oregano, ½ teaspoon salt, and ¼ teaspoon pepper.
Arrange tuna steaks in shallow baking dish and pour ½ cup of the vinaigrette over them. Turn tuna to coat. Cover and refrigerate for 15 minutes.
Meanwhile, combine the remaining vinaigrette with the tomatoes, capers, and arugula. Set aside.
Heat grill or grill pan to medium-high. Grill the tuna, 3 to 4 minutes per side for medium-rare. Serve topped with the salsa.