I wouldn't know, I've never grown an orange in my life. But my sister, who has, reckons it's stress-related, either via fluctuating temps or an irregular moisture supply, or even both. And there's not much you can do about it except, perhaps, monitor the moisture supply, keeping it regular. I suppose - as with tomatoes, which I do know about - a sudden influx of moisture will expand the fruit's innards, splitting the skin that is unprepared for the expansion. I never knew why my sister told me these things. Now I do. So I can pass them on!

We used to force some of our orange trees by letting them go without water for a while. They'd drop most of thier main crop and come back into flower after a good watering. It would give us a second crop for late season use.

So I'd say that this is your problem citrus love heaps of water. However, they usually just drop the fruit if they get stressed like that, not split them.

ciao
Karl

Jan, you could pick them green but they won't develop anymore flavour. Oranges stop ripening once picked. They will mature, and may even colour up, but don't expect their flavour to be much improved from the time you pick them.