Rinse the sardines under cold water, then pat them completely dry with paper towels, inside and out. Check that they're scaled and gutted, and lay them in a shallow dish.
Zest 1 lemon into a small bowl. Add 3 tbsp olive oil, the grated garlic, oregano, salt, and pepper, and stir. Spoon the marinade over the sardines and turn them to coat, including inside the belly. Cover and refrigerate for 15 minutes.
Set a 30 cm / 12-inch cast-iron grill pan over medium-high heat for 5 minutes, until a drop of water sizzles and vanishes at once. Brush the ridges lightly with 1 tsp olive oil.
Lay 4 sardines in the pan with space between them. Grill for 3 minutes without moving them, until the skin is blistered and releases from the pan, then flip with a thin spatula and grill another 3 minutes. The flesh should be opaque and pull away from the bone, or reach 145 F / 63 C if you check with a thermometer. If a sardine sticks, give it another 30 seconds before trying again.
Move the first batch to a warm plate, then grill the remaining 4 sardines the same way.
Squeeze the juice of the zested lemon over all the sardines, drizzle with 1 tbsp olive oil, and scatter on the parsley. Serve right away with the lemon wedges, within 10 minutes of leaving the pan.