The difference between grit and oyster shell is that oyster shell is consumed for the calcium content to be used in production of eggshells. Grit is small stones and large grains of sand that is either provided by humans or gathered by free ranging chickens to aide in digestion or more precisely held in the gizzard or ventriculus to help grind up or crush food before it enters the stomach. Grit is a necessary component of their diet and will develop problems with lack there of unless they are fed only layer mash. While calcium is an important part of shell development and a good layer feed will provide enough for most birds,sometimes people will give added oyster shell or egg shell free choice if some birds require more. They are mutually exclusive and not the same.