Yes, additional calcium on the side with their diet, even if they're on a layer feed will allow the girls that do not absorb calcium as easy as other hens (it's genetic) will enable them to take more when they need it. Chickens just seem to know. Oyster shell is not available where I live. The feed store offers finely ground eggshell instead, my girls love it and it's very inexpensive, I put a bowl of it next to their regular feed. You can also toss them sardines, shrimp shells with or without the meat in them (raw or cooked), their own egg shells broken up and unrecognizable and DARK leafy greens (the darker the better) for calcium boosts. Remember that these treats should never be more than 10% of their diet as over that percentage would defeat the purpose of feeding a BALANCED chicken feed. Some farmer/scientist somewhere worked out how much of the various nutrients chickens need, plus or minus 10% and formulated a grind that would fulfill a chicken's needs; take advantage of their efforts
