Preheat grill. Brush eggplant with oil and grill about 4 minutes on each side, or until soft with visible marks. Meanwhile, in a bowl, whisk together the olive oil, garlic, chile powder, oregano and vinegar until emulsified.
Remove eggplant from grill and cut into 1/2 inch dice. In a bowl, combine eggplant with onion and toss with dressing. Crumble feta over salad.