If using wooden skewers, soak them overnight to prevent flaming on the grill.
For meat: Skewer 5 or 6 pieces of lamb onto each of several skewers.
In a bowl, combine olive oil, lemon juice, oregano, and salt. Pour into a shallow baking dish. Lay lamb skewers in dish, turning to cover all sides. Cover and marinate in refrigerator at least 4 hours or overnight.
Grill lamb over coals or broil in oven until medium-rare or medium. (Lamb cooked “well” will be tough.)
For dressing: In a bowl, combine garlic, olive oil, lemon juice, feta, vinegar, oregano, and salt. Stir to blend.
To assemble: Cut pitas in half or cut off top third of pita. Put one skewer of lamb into each pita. Add onions, tomatoes, and feta. Add dressing, to taste. Best if all used after 1 day.