look up any word, like plopping:
 
1.
Abbreviated form is OMg. Originally created to make fun of people who say OMG, so instead you would say "oxygen magnesium!!"

Unfortunately, it was later discovered that it would actually be magnesium oxide (MgO) but the joke still stands.
Some girl: Look at her shoes, aren't they ugly??
You (overhearing the above): Oxygen magnesium!! That's horrible!!
by Kevin December 06, 2003
 
2.
the long (hand) way of saying or writing OMG or Oh My God/gosh/goodness.

Most often used in the presence of highly religous people who would be offened by the use of the phrase, by people who think they are smart/cool/a hipster, or by new chemistry students who are trying to memorise atomic symbols.

Can also be spelt without capitol 'm' or as two words.
person 1- "Oh My God!"
person 2- "OMG!"
person 3- "OxygenMagnesium!"
person 1- "you just started chemistry today, didn't you?"
by SandraMuffins March 05, 2013