I've done this before, so guaranteed -- will pay appropriately for
effective answer. I need the spelling of coworker. Is it hyphenated or
not? This is a more difficult question than it appears. Yes, it is...
and no it isn't hyphenated. Merriam-Webster and the American Heritage
Dictionary concur: no hyphenation (coworker). I have (coincidentally)
a coworker who insists we should be using the AP Style, which says
co-worker is hyphenated. I want to win this argument, but know it's
not a matter of right or wrong but of gradation. My point to her is
that we follow the mainstream nomenclature when using the language,
which is constantly evolving. If it is mostly used without
hyphenation, bing, I win; if not, she wins -- based on my agreement
with her (she reports to me) that we use the AP Style in my
Communications Department. I look forward to your answer.

Thanks for this stimulating question, 1722.
I've researched several sources on the Internet about this word... if
you have a wager with your friend, I'd consider it a draw. Both forms
are acceptable.

Co-worker
http://dictionary.reference.com/search?q=co-worker

Coworker
http://dictionary.reference.com/search?q=coworker

In fact, if you will look at the references I list below, some insist
on "coworker", some "co-worker".

I assume that styles like that of the Associated Press are set for
consistency in all of their publications. Such reputable publications
would prefer that they use "co-worker" in all of their articles, since
if they use that one time and then "coworker" in another instance, it
would smack of inconsistency, and some readers would interpret it as
poor management. It's nitpicking, but some people are really sensitive
to this, both among readers and among Associated Press editors.


However, I do find some leaning toward "coworker":

Wilbers: Spelling Compound words
http://www.wilbers.com/part24.htm
- The writer of this site examined the Associated Press Style book and
other references, and concluded that neither version of the word is
wrong. Note though that the writer put "co-worker" in parentheses;
probably meaning it's less often used.

These are the grammar, usage, and style errors seen most commonly in
written materials - JP Schoemer Communications
http://www.jpschoemer.com/MostCommonErrors.html
- Here the rule is that the hyphen is used if you get double vowels,
thus, "ultra-adept" and "anti-ignorance" ("cooperative" and
"preeminent" are among the exceptions that became accepted over use).
Under this rule, the hyphen would then be unnecessary for "co" and
"worker".


However, for brevity and ease of writing, I'd go with "coworker"; the
extra dash can be saved.


Here's another observation. "Coworker" is the American English use,
while "co-worker" is British English:
Stewart's Corner - Some tips on hyphenation 
http://www.ntnu.no/universitetsavisa/1098/stewart.html


Instances of "coworker" (They do not all discuss the word though):

Know Thy Coworker - MIT article
http://www.ai.mit.edu/lab/olympics/2002/events/coworker/index-old.html

The Grammar Logs -- Number Five Hundred Twenty-Eight
http://webster.commnet.edu/grammar/grammarlogs4/grammarlogs528.htm

Lesson Plan Library - Unit 15, Prefixes
http://www.lessonplanlibrary.com/zbpdfs/slessons/SpellingConnections_g5_te1.pdf

Crazy Colour.com - Telling a Coworker They Have a Personal Hygiene
Problem
http://www.crazycolour.com/os/coworker_05.shtml

Medical Library Association: Style Manual: Spelling, Usage
http://www.mlanet.org/publications/style/stylspell.html

Theological Studies Spelling Sheet
http://www.ts.mu.edu/contributors/abbreviation_sheet97.htm

Instances of "co-worker":

CMI/Tips/Grammar Crammers
http://www.cmiglobal.com/tips_crammers.html

Grammar Rangers.com, it's where common sense is just a recommended
requirement - in the article "Father lives..."
http://grammarrangers.unsta.com/

How to Win the Grammar Game
http://www.ossweb.com/vp-intro.html

EMU Publications Style Guide: Alphabetical listing
http://www.emich.edu/styleguide/lus/lubc.htm

Garbl's Editorial Style Manual: C (see entry on "co-")
http://garbl.home.attbi.com/stylemanual/c.htm

http:// mitpress2.mit.edu/e-journals/Computer-Music-Journal/Documents/auth/CMJ_Spelling.pdf

C: Alphabetical Listing: Usage: Spelling, Usage and GSC
Recommendations: Guide to Authors
http://www.nrcan.gc.ca/ess/pubs/guide/spell/usagec_e.html

Others:
TSC - Tech Support Comedy! - Co-Worker Types
http://www.techcomedy.com/text_con.php?type=co_worker_type
- You will see both "coworker" and "co-worker" here.

Google search terms: 
co-worker spelling
co-worker grammar
co worker hyphen

Dictionary.com:
Co-worker
Coworker

I do notice that "co-worker" is preferred for many university
publication styles, mentioned above. There are others though, such as
the medical stylebook, that prefer "coworker". I believe it all
depends on what the people of the publication or industry are
accustomed to. Maybe you could call a vote in your department on what
version of the word to use, or settle on a stylebook you will all
use... choice of the stylebook is usually what determines how
communications department people will write their stuff.

I hope this has been a most helpful answer. If you have any problem
with it, do please post a Request for Clarification and I shall
respond. Thank you.

There is no single authoritative source for proper English usage. If
your company has decided to use AP style, then "co-worker" is right,
because that's AP style. If you go by the dictionaries you have, it's
"coworker."

The Economist's Style Guide suggests that the general rule should be
to insert a hyphen but that some prefixes, especially those of one
syllable (such as "co") can be used to form single words e.g.
"coworker".

:) jem-ga

Personally, I prefer the hyphenated version.

Otherwise I see a "cow" at the beginning of the word, and for a moment
my mind goes "Huh? Cow orker? What the heck is an orker?"

Dilbert fans, feel free to chuckle.

Scott Adams, the cartoonist who gives us Dilbert, coined the term
cow-orker for those unpleasent colleagues so many of us must deal with
on a daily basis.

It appears that "coworker" is more popular on the web than
"co-worker."

A simple google search turns up 142,000 entries for "coworker" and
127,000 entries for "co-worker."  (Actaully, the number for
"co-worker" is 269,000 - but that includes all entries for "coworker,"
so subtract 142,000 from 269,000 and you get the 127,000, or 15,000
more entries for "coworker.")

One can infer that "coworker" is used more popularly than "co-worker"
and, therefore, is more generally accepted as correct.

- ddelphi

Agree with the oracle.  Our language is slowly being whittled down to
an economy of keystrokes.

A load of thanks for the full-star rating and the tip, 1722! :)