Gonna have to disagree here. Oyster shell is not readily soluble in water unless it's quite acidic. You don't see oysters in the sea dissolving do you?
Further, take some whole corn which is about the hardest grain most birds will encounter. Soak some in warm water along with an equal amount of oyster shell for a few hours then pinch them between your fingernails. The corn will have softened considerably, the oyster shell not at all.
Oyster shell may not be the most optimal kind of grit, but it can and does work. I feed no grit at all to any of my birds yet I do feed a considerable amount of whole corn, wheat, and oats. Ground oyster shell and sand is the only grit material my birds have and it gets the job done.