“A guy walks into a bar…”  is a typical form of what has been called the “bar joke.” An Oxford comma version is:
 
“An Oxford comma walks into a bar. Orders a gin, and tonic.”
 
The joke was posted on Twitter on July 30, 2013 by NeinQuarterly.
 
“An Oxford comma walks into a bar. It sits, drinks, and leaves”—the Oxford comma joke with a different punchline—was posted on Twitter on January 19, 2013.
 
A lengthy, related joke, “An em-dash, a semi-colon and an Oxford comma walk into a bar…,” was printed in the comic strip Two Pedants on February 26, 2013. “Jeff, a semicolon, and an Oxford comma walk into a bar. They both had a great time” is an Oxford comma joke from 2016.
     
     
Wikipedia: Serial comma
In English language punctuation, a serial comma or series comma (also called Oxford comma and Harvard comma) is a comma placed immediately before the coordinating conjunction (usually and or or) in a series of three or more terms. For example, a list of three countries might be punctuated either as “France, Italy, and Spain” (with the serial comma), or as “France, Italy and Spain” (without the serial comma).
 
Twitter
Ms. Infertile to you
‏@Ms_Infertile
An Oxford comma walks into a bar. It sits, drinks, and leaves.
#GrammarJokes
7:07 PM - 19 Jan 2013
Berlin, Germany
 
Twitter
Andrea Reimer 惠綺文
‏@andreareimer
RT @neinquarterly: An Oxford comma walks into a bar. Orders a gin, and tonic.
2:45 PM - 30 Jul 2013
 
Twitter
Ano Tidaish
‏@tidaish
LOLOLOL! “@NeinQuarterly: An Oxford comma walks into a bar. Orders a gin, and tonic.”
2:56 PM - 30 Jul 2013
 
Twitter
Nein.
‏@NeinQuarterly
Two independent clauses walk into a bar. Order a comma. Leave spliced.
3:32 PM - 30 Jul 2013
 
reddit
An Oxford comma walks into a bar… (self.dadjokes)
submitted August 4, 2013 by bradfink2
Orders a gin, and tonic.
 
Facebook
Shit Academics Say
February 18, 2016
An Oxford comma walks into a bar, orders a gin and tonic and ponders the need for existence in a world of unambiguous sentence construction.
 
Twitter
Spooky Muscular Dad
‏@melynagh
@rcanny_31 an Oxford comma walks into a bar, eats, shoots and leaves
12:28 PM - 18 Feb 2016