Preheat a non-stick pan over medium heat.
In a bowl, combine olive oil, lemon juice, minced garlic, dried oregano (if using), salt and pepper to create the marinade. Ensure that each mackerel fillet is well coated with this mixture.
Place the mackerel in a single layer on top of the pan.
Cook for about 3-4 minutes per side, or until fish flakes easily when pierced with a fork and has reached an internal temperature of around 145 degrees Fahrenheit (63 degrees Celsius).
Remove from heat once cooked. Serve immediately.