Toss potatoes, carrots, celery, and onion in a bowl with 1 - 2 tbsp of olive oil and a little salt and pepper. Stir to coat. Dump into a large crockpot.
Place Pork tenderloin on top of vegetables. Drizzle with another 1 -2 tbsp of olive oil top with garlic, oregano, salt, and pepper.
Cover and cook on high for 5-6 hours depending on the size of roast or low for 8 -10 hours.
Use a meat thermometer to ensure the pork tenderloin is done before serving. Serve with veggies and a side salad.