Acid soak and rinse. Star San/water, or even vinegar/water if that's all you have to work with...
Yep, this. ^^^
You must have hard water? It is a reaction between the oxyclean and the hard water. Can't remember the specifics, but I have had the same thing happen to me before I installed my water softener. Acid will remove it. Make a nice strong solution of star san and it will come right off.