They are at their absolute best when alive. No chemical breakdown has happened at all. Starting at the moment of death, all meat, of any kind, starts many chemical processes that break down the meat, all the way to rotting.
That said, when packaged, *supposedly* they are under tight time and handling restrictions that are supposed to extend their safe-to-eat life-spans as far as possible.
If you've never tried this, it's sure worth the effort:
Shuck and pick oysters. Wash to remove any particles of shell or sand in shell.
Wrap each oyster in a half slice of thin-cut bacon.
Bake until bacon is cooked to your liking.
Quarter a piece of bread and butter it with a butter/garlic/chive mix.
Put one wrapped oyster between two quarters of bread and gobble it up!
Four Yums on the Yum Meter!
Warning; this is a cholesterol/fat meal that is NOT good for you, but try it anyway....