ZOMG predates 4chan, which launched in October 2003 and seems to have been used as a sarcastic way to say "Oh my God!" .
From a since deleted Wikipedia article:
ZOMG is a Derivative of the Internet shorthand term for "Oh My God", OMG. The term is commonly used to express humorously overwrought excitement and as a way of mocking internet slang as a whole, and is most likely representative of pushing the Shift key over-zealously and sloppily, thus pushing the "Z" key in the process, on a QWERTY keyboard. It has appeared on various websites and message boards, often associated with a "witty" remark or trolling, and was spotted in use as early as 1996. It is reasonably widespread today in the online Anime and gaming communities, and is often used in its more ironic sense on internet pop-culture forums such as Fark.
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Usage
ZOMG is often followed by a variation of LOL or by an excessive series of exclamation points trailing into a numerical "1". This last convention implies the typist's ineptitude in creating a simple Exclamation mark on a QWERTY keyboard. A common addition to the previous scenario is to append the word "one" after the numerical "1", e.g., "ZOMG!!!!!!1111one". On some message boards, this is sarcastically extended to phrases such as "ZOMG!!!!1111onetwoeleventy111". In some cases typists will go to extreme lengths such as typing "ZOMG!!!11!1one!1cos(0)". In each case, the (self-depreciating) implication is that the typist lacks the wherewithal to type correctly.
In gaming communities, ZOMG is often followed by "hax", and cries of "ZOMG hax!" and the like may be heard when a player does anything especially impressive, implying (ironically or otherwise) that the player must be hacking the game to do so well.
Another meaning of the word suggests a usage as an odd child of the two words "Zing" and "OMG". This implies a more humorous nature to the word itself without the necessary implications of surprise that come with the acronym "OMG". Others suggest that "ZOMG hax" originally spawned in StarCraft, if a player lost quickly to a Zerg player, causing the losing player to think his opponent hacked because he won so quickly. Thus, "Zerg, OMG, hax" quickly because "ZOMG, hax."
Popularity
This slang word was not made popular on the Internet at large until its "reinvention" in 2003. ZOMG spread from a personal joke between James Curbo and friend Matthew Milan to the Raspberry Heaven Internet community, from there to the Something Awful Forums, LUElinks Forums, and popular image board 4chan upon its opening in October 2003, and from these places spread to the Internet community at large.
From a 2005 revision:
Origin
Although often cited as an invention of USAF officer James Curbo in 2003, ZOMG appears in a USENET post from 1999 and its use in some online MUD communities has been cited as far back as 1996. Some have suggested that its inventor in 2003 had seen ZOMG somewhere before without realizing it, while others consider it a case of parallel invention, as with Richard Jordan Gatling's invention of the screw propellor for steam boats only to discover that another man had had the same idea and beaten him to it.
In any case, the word was not popular on the internet at large until its reinvention in 2003. "ZOMG" spread from a personal joke between James Curbo and friend Matthew Milan to the Raspberry Heaven internet community, from there to the Something Awful Forums, LUElinks Forums, and popular image board 4chan upon its opening in October 2004, and from there to the internet community at large, particularly among anime and gaming fans.
ZOMG's popularity can be attributed partially to the difficulty in expressing emotion via the internet. Since the phrase OMG ("Oh My God") is often taken seriously when the speaker means it as a joke, the addition of a Z to the acronym gives it a certain ridiculousness, since few words begin with the letter Z would fit the acronym.
It makes sense to assume that ZOMG came from a typo of "OMGz," -z being a common intensifying suffix used in leetspeak.
And from another 2005 revision:
ZOMG also possibly originated from BBS chatroom emote commands such as zlaugh, zcry, etc.
I also found another copy of the ZOMG FAQ, the contents are the same but this one has a copyright notice:
Copyright (c) 2003 Matthew Milan and James Curbo
So it was written by both of them. I also found this from 2005:
zomg
- A social experiment started by Matthew Milan (Rick_Feynman) and James Curbo (hannibal)in January 2003 to explore methods of speading and popularizing internet memes. It was also a mocking jab at people who practiced "leet-speak".
Originally meant to be a more intense version of 'OMG' - who knows what it means now.
Originally 'launched' in two places, the Toronto Raptors Usenet Group and the Raspberry Heaven Direct Connect hub. From there it spread into the anime fan-subbing community, the Something Awful forums and the 4-chan Image Boards.
The oldest reference I can find is by Jonathan Seda on the alt.fan.nb Usenet group for Nintendo gamers, 24th April 1999. Here's a snippit of a presumably fictitious chatlog:
KREMIS: oh no!!!
KREMIS: BASTAR!D!!! fag!!
KREMIS: Go away!!!!! NOW PLEAEE!!!!!!!!!!!!!!EEEEE!!!
KREMIS: ZOMG!!!!!
KREMIS: FFF
I also found ZOMG in signatures of forum posts dating back to 2001. Here's syntrix's signature, 28th September 2001, interesting for the zombie mention and ASCII art:
MOEW: ٩(●̮̮̃•̃)۶ Zomg Zombies ٩(-̮̮̃-̃)۶ ٩(-̮̮̃•̃)۶ ٩(×̯×)۶ ¯\(°_o)/¯
Jorji-'s signature, 24th January 2002:
zomg she finally got myspace lawls
spin's signature, 16th July 2002:
zomg, no more sites!
stu's signature, 26th December 2002:
ZOMG
Finally, as of last month, ZOMG has been added to Oxford Dictionaries Online, but doesn't help us much:
Origin:
early 21st century: perhaps in reference to fast or careless typing of OMG, during the course of which the shift key is missed and the 'z' key pressed instead.