Place the tuna between 2 pieces of wax or parchment paper and pound out to 1/8 inch thick. Season both sides with salt and pepper.
In a shallow bowl, combine the breadcrumbs, orange peel, parsley and garlic.
On a work surface, cut off the ends of each orange. Working with 1 at a time, stand the oranges upright and cut off the peel in strips from top to bottom. Working over a bowl, cut between the orange membranes to release each segment. Transfer the segments to a work surface, chop, then return to the bowl. Add the scallions and oregano, drizzle with EVOO and season with salt and pepper.
Coat the fish with the seasoned breadcrumbs, pressing to adhere. In a large nonstick skillet, heat 2 tablespoons EVOO, 2 turns of the pan, over medium-high heat. Add 2 tuna cutlets and cook, turning once, until golden, 3 to 4 minutes; transfer to a platter. Repeat with the remaining fish, adding more EVOO as needed.
In a medium bowl, toss the arugula with the lemon juice and a drizzle of EVOO; season with salt and pepper. Top the fish with the orange-and-scallion salsa and serve with the salad.