“OJ” for “orange juice” is now widely known, but it started as soda jerk slang. “‘OJ’ is orange juice” was printed in an Associated Press story in February 1934. “Orange juice, ‘O. J.’” was printed in The Sunday Sentinel-Star (Orlando, FL) on September 3, 1939.
 
“TJ” for “tomato juice” was also used, but is among several other initialisms that failed to become popular.
 
 
(Oxford English Dictionary)
OJ n. (also o.j.) colloquial (orig. U.S.) orange juice.
1942   L. V. Berrey & M. Van den Bark Amer. Thes. Slang §816/59   Fruit juice,..O.J., sunkissed, orange juice.
     
27 February 1934, Casper (WY) Tribune-Herald, “Students Give New Meaning to the Alphabet,” pg. 6, col. 7:
LOS ANGELES, Feb. 27.—(AP)—(...) It’s the new vernacular of the work-your-way students who wait on the trade at the Westwood cafeteria of the University of California at Los Angeles and who unofficially constitute the They-Don’t-Speak-Our-Language-Association.
 
The waiter’s shout of “JD” means Jersey dark—a chocolate milk shake to you. “OJ” is orange juice, “SO” limeade (squeeze one), and “CO” lemonade (cut one). “GS” is a peanut butter sandwich (goober san), and “AC” American cheese sandwich.
 
3 September 1939, The Sunday Sentinel-Star (Orlando, FL), “Inside Dope on Lake County” by Ormond Powers, pg. 17, col. 5:
The lingo of the drug store boys has always seemed to us nothing short of marvelous. With the help of Guy Neal of Leesburg, who seems to be perfectly at home with simple syrup and cracked ice, we have prepared a list so you can marvel, too.
(...)
Many drinks are called by their initials: grape juice, “G. O.” (after Ola); tomato juice, “T. J.”; ginger ale, “G. A.” (also called firewater); orange juice, “O. J.”
   
31 January 1942, Chicago (IL) Daily Tribune, “White Collar Girl” by Ruth MacKay, pg. 13, col. 2:
Soda Fountain Slanguage.
“Burn one”—chocolate malted milk.
“O. J.”—orange juice.
 
September 1949, Hearst’s International Combined with Cosmopolitan (New York, NY), “Gimme a 51 and a Parlez-Vous,” pg. 82, col. 3:
Here’s a list which may help you solve some of the mystic language which is shouted behind the soda fountain:
(...)
2. O.J. (Orange juice)
     
10 June 1951, Miami (FL) Sunday News, “A Language Of Their Own” by Harvey Keeler, Miami Sunday News Magazine sec., pg. 5, cols. 2-3:
“O.J.” for orange juice is universal. At one fruit stand on Miami Beach where people like to see their oranges freshly squeezed, some of the younger fry amuse themselves by calling “One O.J.” when the clerks are particularly busy, then disappearing into the crowd to watch the bewildered clerk try to find who ordered the orange juice.
   
22 April 1956, Shreveport (LA) Times,  “Soda Jerk Jargon—A Dead Slanguage?” by Dewey Finley, pg. 3-F, col. 4:
GLOSSARY
(...)
OJ—Pure orange juice.
 
9 August 1958, St. Petersburg (FL) Times, “Have A Lingo All Their Own” by Nancy Osgood, pg. 4-B, col. 7:
ALSO—‘shot” for small Coke—“sweetie” for sweet milk—“churn” for buttermilk—“J-D” for chocolate milk—while a small glass of orange or tomato juice becomes “a stubby O-J” or “a stubby T-J.”
   
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jack
@jack_ludwick
on this edition of “wow it took jack too long to figure that out” people call oj simpson “juice” because OJ is slang for orange juice!!!
12:14 AM - 22 Mar 2017