“A guy walks into a bar…”  is a typical form of what has been called the “bar joke.” A chemistry joke is:
 
“Oxygen and Magnesium walk into a bar. The bartender says, ‘OMg!’”
 
The symbols for the chemical elements Oxygen (O) and Magnesium (Mg) combine for “OMg.” OMG means “Oh My God.”
 
“If you put Oxygen and Magnesium together, you get OMG!” was posted on Twitter on November 4, 2009. “Q: What did the bartender say when oxygen and magnesium walked into his bar? A: OMg” was posted on Twitter on January 30, 2011. The joke has been printed on many images.
 
“Oxygen, Hydrogen, Sulfur, Sodium, and Phosphorus walk into a bar…” is a similar joke.
         
   
Twitter
Devon
‏@DevKitty
Becca: “If you put Oxygen and Magnesium together, you get OMG!’
2:56 PM - 4 Nov 2009
 
Twitter
Jisha Puniyani
‏@jishapuniyani
Q: What did the bartender say when oxygen and magnesium walked into his bar? A: OMg
11:28 AM - 30 Jan 2011
 
Twitter
Hayley Whitchurch
‏@haywhitchurch
oxygen and magnesium walk into a bar. OMg
2:26 PM - 12 Jul 2011
 
Twitter
Cameron
‏@CRJG_
Oxygen and Magnesium walk into a bar and the bar man says “OMg”
6:01 PM - 12 Jul 2011
 
Twitter
G★
‏@gbuck
Oxygen and Magnesium walk into a bar…Bartender drops everything and says “OMg”
10:57 AM - 16 Aug 2011
   
WeKnowMemes
I Heard Oxygen And Magnesium Are Dating…
– DECEMBER 24, 2012
I heard oxygen and magnesium are dating – I was like OMG
     
ThatVoiceInsideMyHead
February 9, 2013
Top Ten Chemistry Jokes! 
(...)
3. Did you hear? Oxygen and magnesium are like totally going out it’s like OMg.
 
Google Groups: hackerspace_brisbane
and now for something completely different…
simon.shea
7/6/13
(...)
9. When I heard that oxygen and magnesium hooked up I was like OMg.
 
Google Books
More One Liners, Jokes and Gags
Compiled by Grant Tucker
London: Biteback Publishing Ltd
2013
Pg. ?:
When I heard that oxygen and magnesium hooked up, I was like OMg.
 
Reddit
did you hear Oxygen and Magnesium got together
submitted November 24, 2014 by MadGo
OMg
   
Google Books
Extremely Embarrassing Dad Jokes:
Because Dads don” t know when to stop
By Ian Allen
New York, NY: Pavilion Books
2015
Pg. ?:
Did you hear that Oxygen and Magnesium have got together?
OMG!
 
Google Books
Humour and Jokes
By Arijit Ghosh
Smashword
2015
Pg. ?:
Did you hear oxygen and magnesium dating together?
OMg!!
 
Google Books
Bad Science Jokes
Jokes For Alkynes of Scientists
By Melissa Miller
Lulu Publishing (Lulu.com)
2016
Pg. 12:
Oxygen and Magnesium went on a date. OMg!
   
Reddit
Oxygen and Magnesium walk into a bar…
submitted October 14, 2016 by Insert_Release
OMg!