Halve and steam potatoes, using a basket steamer, until just tender, about 8 min. When cool enough to handle, use a small spoon or melon baller to scoop the flesh out of each potato into a mixing bowl. Place potato shells on a parchment paper-lined baking sheet and set aside.
Mash potato flesh with a fork. Mix in half the Parmesan, 2 tbsp (30 ml) oregano, 1 tbsp (15 ml) olive oil, yogourt, lemon zest and juice, salt and pepper, until well blended. Spoon the filling back into the potato shells evenly. (If making ahead, potatoes can be wrapped with plastic at this point and refrigerated until needed.)
To serve, preheat oven to 450°F (230°C). Top the filled potatoes with the remaining Parmesan cheese and a drizzle of the remaining olive oil. Bake for 12 to 15 min. or until golden brown and crisp. Garnish with remaining oregano and transfer to a platter and serve warm.