We are often asked, "why do you spell "G-d" without the O?
The reason very simple; we as Jews do not take any name lightly, but in the case of the Creator, Blessed Be He, His Name is so Holy, we treat that Name with the upmost care. It is in keeping with the Commandment; "Thou shall not take the L-rd Thy G-d's Name in vain.
Judaism 101 has a very good article that I have included in this journal and quote them as the soucre....
Writing the Name of God
Jews do not casually write any Name of God. This practice does not come from the commandment not to take the Lord's Name in vain, as many suppose. In Jewish thought, that commandment refers solely to oath-taking, and is a prohibition against swearing by God's Name falsely or frivolously (the word normally translated as "in vain" literally means "for falsehood").
Judaism does not prohibit writing the Name of God per se; it prohibits only erasing or defacing a Name of God. However, observant Jews avoid writing any Name of God casually because of the risk that the written Name might later be defaced, obliterated or destroyed accidentally or by one who does not know better.
The commandment not to erase or deface the name of God comes from Deut. 12:3. In that passage, the people are commanded that when they take over the promised land, they should destroy all things related to the idolatrous religions of that region, and should utterly destroy the names of the local deities. Immediately afterwards, we are commanded not to do the same to our God. From this, the rabbis inferred that we are commanded not to destroy any holy thing, and not to erase or deface a Name of God.
It is worth noting that this prohibition against erasing or defacing Names of God applies only to Names that are written in some kind of permanent form, and recent rabbinical decisions have held that writing on a computer is not a permanent form, thus it is not a violation to type God's Name into a computer and then backspace over it or cut and paste it, or copy and delete files with God's Name in them. However, once you print the document out, it becomes a permanent form. Thatis why observant Jews avoid writing a Name of God on web sites like this one or in BBS messages: because there is a risk that someone else will print it out and deface it.
Normally, we avoid writing the Name by substituting letters or syllables, for example, writing "G-d" instead of "God." In addition, the number 15, which would ordinarily be written in Hebrew as Yod-Heh (10-5), is normally written as Tet-Vav (9-6), because Yod-Heh is a Name. See Hebrew Alphabet for more information about using letters as numerals. Pronouncing the Name of God
Nothing in the Torah prohibits a person from pronouncing the Name of God. Indeed, it is evident from scripture that God's Name was pronounced routinely. Many common Hebrew names contain "Yah" or "Yahu," part of God's four-letter Name. The Name was pronounced as part of daily services in the Temple.
The Mishnah confirms that there was no prohibition against pronouncing The Name in ancient times. In fact, the Mishnah recommends using God's Name as a routine greeting to a fellow Jew. Berakhot 9:5. However, by the time of the Talmud, it was the custom to use substitute Names for God. Some rabbis asserted that a person who pronounces YHVH according to its letters (instead of using a substitute) has no place in the World to Come, and should be put to death. Instead of pronouncing the four-letter Name, we usually substitute the Name "Adonai," or simply say "Ha-Shem" (lit. The Name).
Although the prohibition on pronunciation applies only to the four-letter Name, Jews customarily do not pronounce any of God's many Names except in prayer or study. The usual practice is to substitute letters or syllables, so that Adonai becomes Adoshem or Ha-Shem, Elohaynu and Elohim become Elokaynu and Elokim, etc.
With the Temple destroyed and the prohibition on pronouncing The Name outside of the Temple, pronunciation of the Name fell into disuse. Scholars passed down knowledge of the correct pronunciation of YHVH for many generations, but eventually the correct pronunciation was lost, and we no longer know it with any certainty. We do not know what vowels were used, or even whether the Vav in the Name was a vowel or a consonant. See Hebrew Alphabet for more information about the difficulties in pronouncing Hebrew. Some religious scholars suggest that the Name was pronounced "Yahweh," but others do not find this pronunciation particularly persuasive.
Some Christian scholars render the four-letter Name as "Jehovah," but this pronunciation is particularly unlikely. The word "Jehovah" comes from the fact that ancient Jewish texts used to put the vowels of the Name "Adonai" (the usual substitute for YHVH) under the consonants of YHVH to remind people not to pronounce YHVH as written. A sixteenth century German Christian scribe, while transliterating the Bible into Latin for the Pope, wrote the Name out as it appeared in his texts, with the consonants of YHVH and the vowels of Adonai, and came up with the word JeHoVaH, and the name stuck.