Marinate sardines in olive oil, lemon juice, garlic, oregano, salt, and pepper for 15 minutes.
Preheat grill to medium-high heat.
For salad, combine greens, cucumber, tomatoes, onion, and olives.
Whisk together olive oil and vinegar for dressing.
Grill sardines for 3-4 minutes per side until cooked through.
Toss salad with dressing and top with feta.
Serve sardines with Mediterranean salad.