Always, always offer oyster shell! Chickens are not stupid. If they don't need it they won't eat it.
I have hundreds of chickens, some free range and some are in breeder pens. I offer it to all my chickens that are laying age. Some of my birds consume it quickly and others do not even though I feed a balanced diet with supposedly plenty of calcium in it.
I never realized they needed it so badly until I had not given it for a few months and then offered it again. They gulped it up like candy and started laying a lot better, even those that free range on my pastures. Don't believe what people tell you about a commercialized food. Go by what your chickens tell you, they are the experts.