Have you had problems in the past with the rats and crows being attracted to the oyster shells? So your hens are getting no supplemental calcium right now? I would not think they rat/crows would be attracted to the oyster shells, but I could be wrong.
How many chickens do you have? I have 19 layers and I don't go through too much of it in. I use flaked oyster shells and a small box (5 pounds) last me about 2 months. I never find any undersireable pests eating my oyster shells. My crew free ranges so there is ample opportunity for pests to get near the oyster shells. I have never seen that at all.
Hens do need to be able to get calcium free choice in order to keep their reproduction system in good health. Rooting for you to find a way to make that happen.
Also, oyster shell and grit are two different things. Oyster shell and grit are not interchangable. Grit will not provide needed calcium and oyster shell is not hard enough to be substituted for grit. That they have sand is better than nothing, but they do need hard grit. Grit is usually made up of crushed granite. If your sand is made up of granite (or flint) then you are probably okay. Grit is used to help the gizzard grind food. Oyster shells provide calcium for the production of eggshells. Not having calcium will lead to soft shelled eggs. Even hens on layer feed need supplemental calcium.
I don't give grit b/c my crew free ranges and can find all the smal rocks they want. But I do give oyster shells for calcium.