Oyster Shell

I should think you'd be able to do that. I'd rinse to remove excess salt. Also, studies show that medium-sized pieces are absorbed the most efficiently, not, as you would expect, finely ground. So, smash into bits the size of a fine dice. BTW, I use a rabbit feeder to provide the shells free-choice. I find they eat more when up off the ground and clean in this feeder. You can see a photo here: http://hencam.com/faq/what-to-feed-your-chickens/

Let me say this on crushing your own, GOOD LUCK. A friend of mine gave me a bushel of cherrystones. Which I devoured and also made clams casino and put in the freezer for later use but barely lasted a week. So I got this lame brain idea that I would dry the shells out and then crush them. Far more work that it was worth,at least for the chickens, although I did get one heck of an aerobic work out. What I did was take an old feed bag and put the shells in and ran them over with a home made roller full of concrete to pre-break them, then did the rest with a bottom of a sledge. Took a hour or more to get them down to a edible size with about 3lbs of shell. For $9 I can get a 50lb bag and not break a sweat. Now bear in mind oyster shells are a lot thicker than cherrystones shells.  Next time I might try steamers(soft shelled clams for you mid westerners).

As far shell as grit goes, it does serve as that to a degree but they still need some real grit to break down the shell. Depending how fine your sand is you might have to give them a little something with some fine stones in it.

I live in New England where the the dirt is made of granite bits. I've never had to add grit! Coarse sand should have plenty of grit in it, too.

I know that the courser bits are better utilized.  My chickens seem to know this too.  The last bag of shells I got has a lot of finer pieces in it.  I have had to go out and remove the fines from the shell dish and put fresh in so they would use it.