In a medium bowl, combine pork, oil, 2 tablespoons lemon juice, garlic, and oregano. Season with salt and pepper and toss to combine. Cover with plastic wrap and refrigerate at least 1 hour (or up to 8 hours).
In a small bowl, whisk together yogurt, cup lemon juice, tahini (if using), and 1 to 2 tablespoons water (sauce should have the consistency of heavy cream).
Heat a grill or grill pan to high. Thread pork cubes onto 8 metal skewers. Grill until brown on all sides and cooked through, about 12 minutes. Serve souvlaki with yogurt sauce and lemon wedges.