Preheat the oven to 200C/400F/Gas 6.
Place the celery, stock, onion, garlic and oregano into an ovenproof pan. Bring to the boil, then reduce to a simmer and cover with foil.
Transfer to the oven and cook for 6-8 minutes, or until the celery is tender. Remove from the oven, whisk in the butter and season, to taste, with salt and freshly ground black pepper.
To serve, place the braised celery into a serving dish.