A bed, mattress, futon or even a blanket or sleeping bag on which someone has ejaculated several times, especially when multiple used, dried condoms are left lying on it. This is because oyster is used as slang for ejaculate and used condoms dry up and somewhat resemble oyster shells. An oyster bed will usually belong to a slob or a hogger, almost always male.
Also a term used by fishermen and others to describe areas of seafloor where several oysters (the creatures) reside in their shells.
Pat goes hogging in his bed all the time and doesn't change the sheets. Even worse, he leaves his jizz and dried rubbers in it. He sleeps in an oyster bed and it's disgusting.