This causes a problem in the UK and maybe other places. Separated by a common language. This is another reason it's a good idea to modify your profile to show your general location.
Oyster Shell provides extra calcium for hens that are laying. They need it for their egg shells. Chickens not laying also need some calcium in their daily diet for growth or general body function, but not nearly as much. Layer feed provides about 4% calcium, that should be enough if Layer is all they eat. But if they eat other things, either as treats or from foraging, they may need a calcium supplement. In the UK that is called soluble grit. It could be oyster shell or limestone crushed to a size no larger than a green pea. Some people use their egg shells as a calcium supplement.
Chickens do not have teeth but they often eat things that need to be ground up, grains and vegetation for example. They eat small rocks to use in their gizzards to act as teeth. Oyster shell does not work for that since the digestive juices dissolve it. If your chickens have access to the ground they will find their own grit or you can buy it at the feed store. The stuff you buy is generally granite, waste materials for granite quarries, but they can use practically any rock. The harder the rock the longer it lasts. In the UK that is called insoluble grit.
