Is there an approved or definitive usage for pronunciation of the
numeral zero?
In speaking a series of numerals, such as a phone number, prounouncing
it as "zero" seems best. But if we speak of the year 1908, or the time
as 12:04, it is usually pronounced "oh". Presumably it is idiomatic,
neither is wrong or right.

Oddly, there aren't many online resources that deal with this matter,
other than guidelines for radio transmissions (where the number '0' is
always pronounced 'zee-ro' so that it will not be mistaken for the
letter 'O'.)

Here a distinction is made between British and American usages:

"Zero is the number 0. American speakers use zero in both conversation
and writing.

In British English, zero is normally used only in scientific writing.

The path coefficient is now greater than zero. 

The gravitational pull would grow weaker until we reached the
verycentre of the planet, where it would be zero.

In conversation, British speakers usually say nought or oh. 

Nought point nine 

You arrive at Palma at oh two thirty five." 

http://www.uni-potsdam.de/u/spz/wl/PH001018-31.html

A discussion of the pronunciation of the numeral '0' as it is used in
dates:

http://home.mn.rr.com/wagger/theyear.html

Here's a long newsgroup thread on the subject of '0' as pronounced in
clock times:

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&safe=off&th=d4b216d87ce53bea&rnum=5

I am old enough to recall that when the first James Bond film, "Dr.
No," was released in the United States, quite a few Ian Fleming fans
in the United States were surprised to learn that their favorite
secret agent was not "Oh Oh Seven" (as many Americans would pronounce
'007',) but "Double Oh Seven."

I'm not sure it's quite that standardised in the UK.  I live there and
rarely use "nought".  In fact, the only obvious situation in which I
would use it would be in speaking a decimal, such as 0.9.  On the
whole, I tend to say "oh", but on the telephone, if I'm dicatating a
number to someone, I use "zero" because I think it is easier to
understand. If I want to give a time, I would simply say
two-thirty-five, not oh-two-etc.  However, I would most probably
qualify this with "in the morning" or "in the afternoon" or with "AM"
or "PM".  I would never give the time as 14.35.

then there is 'aught' as in ".30-06" and "Double-aught buck" used in
the gunning community.

To pinkfreud: It is gratifying to learn you are of the age that rememembers 
"Dr. No". Do the British also use "zed"? 
  As always, your comments and answers are marvelous.
Apropos of nothing, do you care to reveal the hidden meaning of your signature
pinkfreud?

Or aught as in the aughts ("There wasn't a Gary consevatory until
aught six" (1906)).

Regards,
luciaphile-ga

knowitall22,

Thanx for the kind words about my comments and answers!

Regarding my screen name, I hope you won't be disappointed to learn
that there isn't very much of a "hidden meaning." At least, it isn't
hidden any more, since I once answered a GA question about it:

http://answers.google.com/answers/main?cmd=threadview&id=80023

~pinkfreud

tehuti-ga: Apparently not standardised anywhere. Didn't know you
reside in the UK. Good show. How would you pronounce 12:05?
  Also, I like your offhand poetry: "On the whole,I tend to say "oh".
OK, it isn't a perfect rhyme.

houstonguy and luciaphile; Indeed, "aught" is another synonym for zero
that I hadn't considered. Isn't it probable that aught is a
contraction of nought?

Hi knowitall22-ga,

According to my Webster's (Ninth New Collegiate), one of the
definitions for aught has an etymology of: "resulting from incorrect
division of a naught" p115

Regards,
luciaphile-ga

Well, this is interesting. The OED has this to say about "ought":
"vulgar corruption of NOUGHT in sense 'cipher.' Then it goes on:
"Prob. originating in an erroneous division of ‘a nought’ as ‘an
ought’; but by many associated with the figure 0 of the cipher, which
they take as the initial O of ought. oughts and crosses, a children's
game with a figure containing nine spaces, which are filled up by two
players alternately with ciphers and crosses, the object of each being
to place three of one kind in a line; = noughts and crosses."

In my younger days I once had an algebra teacher who was very
insistent that we say "zero" in class and never "oh" since, as he put
it, "'oh' is never used in proper society."  I remember talking to him
one day in the hallway and pointing out to him that the recorded voice
the phone company uses (as in "The number you have reached...") said
"oh" and not "zero" (...I'm not certain if it's still this way).  The
teacher didn't really seem to care.

And speaking of noughts, I recommend viewing Peter Greenaway's "A Zed
& Two Noughts":
http://us.imdb.com/Title?0090366

OK then, knowitall, I'd better polish up my rhyme and metre! :)

"I'll tell you though, I tend to say "oh""

We definitely say "zed" in the UK, even though it messes up the Sesame
Street song.

12.05 - I'd say "twelve-oh-five" if I wanted to say it that way, but
if it referred to time, I'd most probably say "five past twelve", or
"five past midday/midnight" as appropriate.

Up North (England I mean), they say nowt to mean nothing and owt to mean anything.

There aught to be a law, but it seems there's nawt.

To all who commented: A marvelous, enlightening series of comments.
Many thanks to all of you for an informative and entertaining
dialogue.

I gave a presentation at a German university once (in English), and
most of the audience were confused by my use of point-oh-five (for
0.05).  I was surprised, as they had no problem with everything else I
said.

Just a quick thought to add to this discussion:

I once knew an American fellow who threw tizzies when someone
pronounced the number zero as "oh." "Oh is a LETTER," he'd say, "not a
number." He never considered, apparently, that "oh" is almost
certainly an abbreviated version of "zerO."

To kriswrite: A most enlightening comment. Never thought of that, and
it explains and justifies pronouncing zero as "oh". Many thanks.
knowitall22