Heat oven to 190C/fan 170/gas 5. Halve the plums and arrange in a snug layer in one large or 4-6 individual shallow, ovenproof dishes. Sprinkle over the sugar and squeeze over all the orange juice. Press a sheet of baking parchment down over the plums and bake for 20-25 mins, until they start to soften.
Meanwhile, heat the butter with the syrup or honey in a pan. Remove from the heat and stir in the oats until they are evenly coated. Remove parchment, roughly spoon oats over the plums, then return to the oven for a further 10 mins, until the topping is crisp and golden and the plums are tender. Serve warm with crème fraîche or ice cream if you are feeling indulgent, or fromage frais or low-fat yogurt if not.