In a bowl, toss the pork with the garlic, oregano, orange zest and juice. Leave to marinate for 10 minutes.
Thread the pork, tomato halves and onion wedges onto skewers until all the pork is used up (reserve the leftover vegetables for the salad). Brush with olive oil and barbecue (or grill) for 8 to 10 minutes, turning once or twice, until cooked through and no longer pink in the middle.
For the salad, combine the olives, spinach, feta cheese and leftover vegetables. Mix the oil and vinegar, season and drizzle over the salad. Serve with the skewers.