I believe in supplying grit at any age. (If they are on the ground, they will find it naturally, but I always put some playground sand in a pile for them anyway)
It is cheap & won't harm them if they don't need it.
It's better to have it for them, than them need it & not have it.
In my opinion, Commercial feed needs grit unless it has it included. The label will say "no grit needed" if it is included in the feed.
They still need to grind the commercial feed up in their crops, if they don't have the grit, the crop can become impacted.
You didn't say how old your chickens are, if they are not of laying age, don't give them oyster shells yet.
Oyster shells can't be substituted for grit. Grit is for grinding up they food, oyster shells dissolve in their crop & is for calcium to produce a good egg shell.
Jean