Many people use the number 0 (zero) rather than substituting '-' for the letter 'o' (such as in JewishWorldReview.com for its columnists) in order to prevent the various editors from putting the letters g and d on separate lines.
When we were writing the soc.culture.jewish FAQ we used the following question.
Note: The story below about Rav Soloveitchik was confirmed by a person who was in the class at Boston's Rambam high school when it occurred.
Section - Question 11.3.1: Writing: Why do some people write "G-d" with a hyphen instead of an `o'?
Based on the words in Deut. 12:3-4, the Rabbis deduced that it is
forbidden to erase the name of G-d from a written document. Since any
paper upon which G-d's name was written might be discarded and thus
"erased", the Rabbis forbade explicitly writing the name of G-d,
except in Holy Books, with provisions for the proper disposal of such
books.
According to Jewish Folklore, G-d has 70 names. However, only one of
these names is the ineffable name, which cannot be erased or
pronounced. Further, of the 70 names, seven may not be erased but they
can be pronounced on certain occasions (such as when reading the
Torah). The other names may be erased and pronounced, but still must
be treated with respect. The Talmud (Shevuot 35a-b) makes it clear
that this prohibition applies only to seven Biblical names of G-d and
not to other names or attributes of G-d, which may be freely written.
The prohibition was later codified by Maimonides (Mishneh Torah,
Yesodei HaTorah 6:1-2). The practice of writing "G-d" is supported in
Shut Achiezer, 3:32, end, where it is endorsed and accepted as the
prevailing custom. Rambam cites Deut. 12-03:04, which states "and you
shall destroy the names of pagan gods from their places. You shall not
do similarly to G-d your Lord." The intent of this is to create an
atmosphere of respect for G-d's name vs pagan gods' names.
As a result of this, people acquired the habit of not writing the full
name down in the first place. Strictly speaking, this only
applies to Hebrew on a permanent medium, but many people are
careful beyond the minimum, and have applied it to non-Hebrew
languages. Hence, "G-d". One explanation is that using G-d is a
reminder that anything which we may say about G-d is necessarily
metaphorical. Spelling out the Name (even in a language other than
Hebrew) would imply that one could speak meaningfully (not just
metaphorically) about G-d.
However, the Shach (Yoreh De'a 179:11) ruled that "God" spelled in a
foreign language does NOT have the status of a "shem" and thus
may be erased, lehatkhila. There is a story about Rav Soloveitchik
(z"l) intentionally writing GOD on the board while teaching a class
and then just as deliberately and intentionally erasing it, so as
to demonstrate by his own example that this was not a halakhically
a problem.
