“A guy walks into a bar...” is a typical form of what has been called the “bar joke.” An orange version is:

“Two oranges walk into a bar. One turns to the other and says, ‘Your round.’”

“Your round” (of drinks) means something different than “you’re round” (a shape). The joke was posted on the newsgroup rec.arts.sf.fandom on June 1, 1997.

“Your round."/"So are you, you fat bastard!” is a similar joke.


Google Groups: rec.arts.sf.fandom
Political views of British fandom? (Was: A great day!)
Steve Brewster
6/1/97
(...)
Two oranges walk into a bar, and one says to the other:
“Your round”.

Google Groups: freeserve.chat
Ain’t it a bastard ?
Cold Ethyl
6/18/04
(...)
Can’t beat a bad joke though.... 2 oranges walk into a bar, one says to the other, “you’re round...”

11 June 2007, The Daily Mirror (London, UK), “Big Bro 8,” pg. 9:
Giving Amanda giant sunglasses, Seany asked her to tell a joke. “Two oranges walk into a bar and the other one goes, ‘Your round, your round’,” she laughed.

Twitter
Mags‏
@Mags2u
two oranges walk into a bar, one says to the other “your round”
2:24 AM - 25 Apr 2009

Google Books
The Laughing Soldier:
The British Armed Forces Jokebook
By Al Murray and Marc Ormrod
Havertown, PA: Casemate Publishers
2010
Pg. 56:
Two oranges walk into a bar, one turns to the other and says, “You’re round.”
Ruth, BFBS Northern Ireland

Google Books
The Mammoth Book of One-Liners
Edited by Geoff Tibballs
London, UK: Constable & Robinson Ltd
2012
Pg. ?:
Two oranges walk into a bar. One says: “Your round.” “Thank you,” says the other.

Twitter
Andrew Jenkins‏
@corbyjenks
Two oranges walk into a bar… One says to the other, “you’re round” #1Pun
12:38 AM - 18 Jan 2018

Twitter
Louise Atkinson‏
@MsLouAtkinson
Replying to @edfringe
2 oranges walk into a bar. One turns to the others and says ‘your round’.
9:08 AM - 19 Jan 2018