Your chickens will no OD on calcium from oyster shell. Chickens are smarter than we give them credit . They know what they need. The oyster shells act as grit also, and then pass thru chicken if not needed. Processed calcium like that which is added into feed gets absorbed much more.I guess my biggest worry is the calcium. They eat a LOT of Oyster shell.
A comparison would be eating corn. (peeps and animals alike.). If it is chewed up finely, there are no traces in the exhaust.