Preheat the oven to 180°C/fan160°C/gas 4. Put the rhubarb in a suitable pie or baking dish (it can be as shallow or deep as you like). Halve the orange around its ‘equator’ and squeeze the juice of one half over the rhubarb. Cut the other half in two, slice each quarter as thinly as you can and add to the rhubarb. Add the sugar and mix everything together.
Make the crumble. Rub the butter, flour and oats together with your fingers until gravelly. Fold in the caster or soft brown sugar. Top the rhubarb evenly with the crumble and bake for 30-40 minutes, or until golden and bubbling.