Grit and oyster shell are two completely different things.
Grit can be anything from sand to small rocks that a chicken eats to help with digestion. The grit is stored in the gizzard, the muscle that uses the grit to grind up the food a chicken has eaten. Once the grit is ground down fine it is flushed out of the system as more is ingested. Normally a free range chicken will source its own grit. A confined bird needs to be given grit.
Oyster shell is a source of calcium that female chickens, pullets and hens, use when they begin laying. If they do not receive additional calcium from an outside source the chicken will pull calcium from its body (bones) to help make the egg shells which is very detrimental to the bird. There is no outside source of calcium a bird can obtain on its own. This must be provided.