It's a cheap insurance policy to offer oyster shells free choice. They often don't consume much, but if it isn't available when they crave it they may look to freshly laid eggs for calcium.
You might want to switch to a different kind of ration. A layer ration will have too much calcium for older birds that don't lay continuously. Too much calcium can cause rough shells, and sluggish birds in my experiences. Feed something like a non medicated starter grower with a separate bowl of...