Oyster shell needs to be crushed into bite sized pieces that will easily go down the esophagus.
But as long as they're not laying yet, that's not important. Oyster shell won't induce laying but replaces the calcium in the medullary bone when they begin to make egg shells.
Were the birds day old chicks in June or were they older? If they were chicks, they reached point of lay some time in late November so aren't really overdue for birds reaching POL in the dead of winter.
Are the pellets, layer pellets, i.e. 4% calcium? If so, how long have they been on layer feed?
I'd switch back to a grower feed until they start to lay, which shouldn't be too much longer now that were past winter solstice. Increasing day length promotes sexual maturity.