Preheat the oven to Gas Mark 5, 190°C, fan 170°C. To make the compote, put the rhubarb in a large shallow baking dish and sprinkle over the sugar and orange juice. Scrape the seeds out of the vanilla pod and add both seeds and pod to the fruit. Mix well so that the seeds are evenly distributed. Roast for 15 minutes until soft.
In a large saucepan, bring the oats, milk and cinnamon to the boil, turn down to a simmer and cook for around 6 minutes. Alternatively, microwave for about 3 minutes on high.
Divide the porridge between 4 bowls and top each one with a teaspoon of brown sugar and the warm rhubarb compote.