Greek Pork Gyros

• 300 g pork fillets, thinly sliced
• 1 brown onion, thinly sliced
• 2 tsp oregano
• 1 tbsp extra virgin olive oil
• 1 tomato, diced
• 0.5 small Iceberg lettuce, shredded
• 2 wholemeal pita bread
• 0.5 cup tzatziki