Preheat oven to 450 degrees F (230 degrees C). Line a baking sheet with aluminum foil and oil lightly with olive oil.
Arrange tilapia fillets onto prepared baking sheet. Place celery, carrots, bell pepper, and red onion on top of fillets. Season with Greek seasoning, salt, and black pepper. Lay a sheet of aluminum foil over the top and fold edges together to seal the packet.
Bake in the preheated oven until fish flakes easily with a fork, about 20 minutes.