Oklahoma Oysters

An “Oklahoma oyster” is a regional term for a “mountain oyster” or “prairie oyster.” The “oysters” are the testicles of an animal.
 
 
Google Books
You All Spoken Here
by Roy Wilder, Jr.
New York: Viking Press
1984
Pg. 89:
Mountain oysters: Lamb, calf, pig and turkey fries; testicles. Any man who won’t eat mountain oysters wears lace on his drawers. “Are these the kind of oysters you put on a rock and crack?” “No. These are the kind you put on a crack and rock.”
Prairie oysters, Oklahoma oysters: Same as above,
   
This Dog’ll Really Hunt:
An Entertaining and Informative Texas Dictionary
by Wallace O. Chariton
Plano, TX: Republic of Texas Press
1999
Pg. 42:
Calf fries: mountain oysters, prairie oysters, or Oklahoma oysters
[The terms are also used to describe lamb, pig, or turkey fries, all of which are the testicles from the animal that are removed during castration, rolled in batter, and deep fried.]