I'm pretty sure this can't be taking The Lord's name in vain since his name is Jesus. I don't think we need to argue the name of the Son of God is Jesus, but let me concrete this a bit more.

I am come in my Father's name, and ye receive me not: if another shall come in his own name, him ye will receive. (John 5:43 KJV)

But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you. (John 14:26 KJV)

So, here we have it, there is one God whose name is Jesus, regardless of the manifestation.

For the sake of everyone who has brought up the word-replacement argument regarding "Oh my God" (e.g. "gosh" vs. "God"), I would like to posit that applying such reasoning to this situation is a slippery slope that ends its downhill slide in absurdity.

Word Replacement

In any given sentence, replacing one objectionable word with another non-objectionable one, when it doesn't alter the meaning of the sentence, is of questionable semantic value. The intent of the speaker is the same, insofar as the meaning of the sentence is concerned.

Now, one might argue, that since both sentences mean the same thing, isn't using the non- (or less-) offensive word is really the same as using the offensive one? If we concede this point, then the problem we have is that the intent of the speaker is what, effectively, is prohibited as opposed to which words he uses.

If I can come up with a different phrase to capture my intent, but lacks the offensive words, does the "word replacement" argument still apply, rendering it prohibited? If, because I don't wish to utter the offensive phrase, come up with some completely new expression, "Flubilly Flashcards!" to use in its stead, is this logically different than replacing "God" with "gosh"? It seems to me that the same logic that equates one will equate the others as well.

If the intent of the utterer of "Oh my God" is to express surprise or exasperation, then using this reasoning to simultaneously prohibit "Oh my gosh," we've effectively prohibited all forms of interjection, including "Wow!", "Ugh!", "Hey!", "Hokey smokes!", etc.

With respect to intent of the third commandment, this necessary conclusion is absurd.

God is prohibiting the irreverent or false/phony use of his name (which the Jews took so seriously that they wouldn't even pronounce it out loud), probably in religious invocation, but not exclusively limited to that scenario.

Does the commandment say "don't speak His name in vain", "don't use His name in vain", or "don't take His name in vain"?

If it says not to take His name in vain, we would first need to consider what it means to take His name. Once that is established we could consider what it would look like to do that in a vain (empty) way.

In a marriage the woman often takes the name of the man, thereby identifying herself as being a part of him / his family. Along those lines, some interpret this commandment to be a warning against identifying yourself as a man (or woman) of God in an empty fashion. For instance, to call oneself a Jew / Israelite / Christian but refuse to follow His ways (love, etc.) would be to take His name in vain.

@a_hardin has a perfectly good explanation of why this expression usually is using the Lord's name in vain. I would make this a comment to his answer, but it's too long. There are only two ways in which I can imagine this phrase would not be using the Lord's name in vain:

1. You are actually addressing or speaking about God.

   Sometimes "Oh my God" is appropriate in this context. Maybe you receive a fantastic birthday gift, and exclaim "Oh my God!" out of gratitude to the Lord--in the same way you might say to a loved one "Oh my dear [Insert name here], I can't believe you gave me this [whatever]!"

   Maybe you're actually upset with God, and exclaiming "Oh my God, (how could you let this happen)?" This wouldn't be using the Lord's name in vain, since it's (technically) a prayer. Just make sure you don't cross the line into cursing the Lord. (I don't think there's anything wrong with being honest with God about your frustration with Him at times.)

2. You're actually saying "Oh my god." (Lowercase g, as opposed to capital G)

   Since this isn't a name of God, but rather a description of a theoretical pagan god, I suppose it wouldn't actually be using the Lord's name in vain. However, in spoken Language, it's impossible to tell the difference, and in written language, one would always assume you mean a big G, and were just lazy or made a typo, or they don't even know the difference. Therefore I would avoid this phrase, too. To me, claiming "But I meant a small g!" is just like saying "Gee!" instead of "Jesus." It's changing the pronunciation/spelling slightly, without changing the meaning in your heart.

Sources:

• Moshe ben Maimon, Sefer ha-Mitzvot, §62

The 62nd prohibition is that we are forbidden to swear a shvu'at shav (a vain oath). The source of this commandment is God's statement (Exo. 20:7), "Do not take the name of YHVH your God in vain."

המצווה הס"ב האזהרה שהזהרנו על שבועת שוא והוא אמרו יתעלה : " לא תשא את שם ה ' אלהיך לשוא." (שמות כ, ז)

[A shvu'at shav is] when one swears that something is the opposite of what it actually is; or, that something exists, when in fact it cannot; or, one who swears in order to violate a mitzvah of the Torah. So too if one swears by an obvious and undisputed fact. For example, swearing to God that anything which is slaughtered will die. Behold, this is also taking the name of YHVH in vain.

והוא שישבע על מחייב המציאות שהוא בהפך ממה שהוא באמת , או על דבר מן הנמנעות שהוא מצוי , או שישבע לבטל מצווה מן התורה . וכן אם נשבע על דבר ידוע שאין עליו מחלוקת ולא ויכוח לשום אדם מן המלמדים , כגון שישבע בה' שכל הנשחט ימות - הרי גם זה נשא שם ה' לשוא.

And the expression of the Mishnah: "What is a shvu'at shav? An oath which contradicts an obvious truth." One who transgresses this prohibition intentionally is punished by lashes. If done unintentionally, he is exempt [from even bringing a sacrifice], as with many other prohibitions, as explained above. And there (that is to say, in Shvu'ot), it is said, "This is the shvu'at shav (vain oath) for which one is lashed if done intentionally and exempt if done unintentionally. The details of this mitzvah are explained there.

ולשון המשנה: "איזו היא שבועת שוא ? נשבע לשנות את הידוע לאדם וגו '." והעובר על לאו זה במזיד - לוקה, ובשגגה - פטור, כשאר חייבי לאווין , כמו שביארנו . ושם אמרו , כלומר בשבועות: "זו היא שבועת שוא שחייבין על זדונה מכות ועל שגגתה פטור", ושם נתבארו דיני מצווה זו.

• Moshe ben Maimon, Mishneh Torah, Sefer Hafla'ah, Hilkhot Shvu'ot, Chapter 1.4-8

4 The first type is when a person took an oath concerning a known matter that was not true, e.g., he took an oath that a man was a woman, a woman was a man, that a marble pillar was gold, or concerning other similar factors.

הָאַחַת, שֶׁנִּשְׁבַּע עַל דָּבָר הַ יָּד וּעַ שְׁאֵ ינוּ כֵּ ן . כֵּ יצַד--כְּגוֹן שֶׁנִּשְׁבַּע עַל הָאִישׁ שְׁהוּא אִ שּׁ ה, וְעַל הָאִשָּׁה שְׁהִ יא אִ ישׁ, וְעַל עַמּ וּד שֶׁלְּשַׁ יִשׁ שְׁה וּא שֶׁלְּ זָהָב; וְכֵן כָּל כַּ יּוֹצֶא בְּזֶה.

5 The second: that one takes an oath on a known matter concerning which no one has a doubt, e.g., one took an oath that the sky was the sky, that a stone is a stone, on two [objects] that they are two, and the like. Even though there is no doubt about the matter for a person of sound mind, one takes an oath to strengthen [the appreciation of] the matter.

הַשְּׁ נִיָּה, שֶׁנִּשְׁבַּע עַל דָּבָר יָדוּעַ שְׁאֵין בּוֹ סָפֵק לְאָדָם בָּעוֹלָם שְׁהוּא כֵּן . כְּגוֹן שֶׁ נִּשְׁבַּע עַל הַשָּׁ מַ יִם שְׁה וּא שָׁמַ יִם, וְעַל הָאֶבֶ ן זוֹ שְׁהִ יא אֶבֶ ן, וְעַל שְׁ נַיִם שְׁהֶם שְׁ נַיִם; וְכֵן כָּל כַּיּוֹצֶא בְּזֶה, שֶׁזֶּה הַדָּבָר אֵ ין בּוֹ סָפֵק לְאָדָם שָׁלֵם כְּדֵי לְצַדַּק הַדָּבָר בִּשְׁבוּעָה.

6 The third is one who takes an oath to nullify a mitzvah. What is implied? One took an oath not to wrap himself in tzitzit, not to put on tefilin, not to dwell in a sukkah throughout the holiday of Sukkot, not to eat matzah on Pesach night, that he would fast on Shabbat and the festivals, or concerning other analogous instances.

שְׁלִ ישִׁ ית, שֶׁנִּשְׁבַּע לְבַטַּל אֶת הַמִּצְ וָה. כֵּ יצַד--כְּגוֹן שֶׁנִּשְׁבַּע שֶׁלּ אֺ יִתְעַטַּף בְּצִיצִית , וְשֶׁלּ אֺ יִלְבּ שֺׁ תְּפִלִּ ין, וְשֶׁלּ אֺ יֵשֵׁב בַּסֻּכָּה בְּחַג הַסֻּכּוֹת , וְל אֺ י אֺכַל מַצָּה בְּלֵילֵי הַפֶּסַח , אוֹ שֶׁיִּתְעַנֶּה בְּשַׁבָּתוֹת וְיָמִים טוֹבִים; וְכֵן כָּל כַּיּוֹצֶא בְּזֶה.

7 The fourth - that one took an oath concerning a matter that he is unable to perform. What is implied? He took an oath that he would not sleep for three consecutive days and nights, he would not eat for seven consecutive days or concerning any analogous matter.

רְבִ יעִ ית, שֶׁנִּשְׁבַּע עַל דָּבָר שְׁאֵין בּוֹ כּוֹחַ לַעֲשׂוֹתוֹ . כֵּ יצַד--כְּגוֹן שֶׁ נִּשְׁבַּע שֶׁלּ אֺ יִישַׁ ן שְׁ לוֹשָׁה יָמִים לַ יְלָה וְיוֹם רְצוּפִ ים, א וֹ שֶׁ לּ אֺ יִטְע םֺ כְּלוּם שִׁבְעַת יָמִים רְצוּפִים; וְכֵן כָּל כַּיּוֹצֶא בְּזֶה.

8 Whenever a person takes an oath in vain by taking one of these four types of oaths, he transgresses a negative commandment, as (Exo. 20:6; Deut. 5:10) states, "And you shall not take the name of YHVH your God for falsehood." If he [takes the oath] willfully, he is liable for lashes. If he does so inadvertently, he is exempt entirely.

כָּל הַנִּשְׁבָּע שְׁבוּעַת שָׁוְא מֵאַרְבַּע שְׁבוּעוֹת אֵלּוּ--עוֹבֵר בְּל אֺ תַעֲשֶׂה, שֶׁ נֶּאֱמָר "ל אֺ תִשָּׂא אֶת-שֵׁם- ה' אֱל הֶֺ יךָ , לַשָּׁ וְא" ( שמות כ ,ו; דברים ה ,י): וְאִם הָ יָה מֵזִיד , ל וֹקֶה; וְאִם הָיָה שׁוֹגֵג , פָּטוּר מִכְּל וּם.

• Philo, The Decalogue, Ch. XXIX, §157

157 By the third commandment, He restrains people from taking oaths, and limits the objects for which one may swear, defining when and where it may be lawful, and who may swear, and how the swearer out to be disposed, both in his soul and body, and many other minute particulars, concerning those who keep their oaths, and the contrary.

• Philo, The Decalogue, Ch. XVII, §82-87

82 The next commandment is, "not to take the name of God in vain." Now the principle on which this order or arrangement proceeds is very plain to those who are gifted with acute mental vision; for, the name is always subsequent in order to the subject of which it is the name, being like the shadow which follows the body.

83 Therefore, having previously spoken of the existence of God, and also of the honor to be paid to the everlasting God, he then, following the natural order of connection, proceeds to command what is becoming in respect of His name. For, the errors of men with respect to this point are manifold and various, and assume many different characters.

84 That Being which is the most beautiful, and the most beneficial to human life, and suitable to rational nature, swears not Himself, because truth on every point is so innate within Him that His bare word is accounted an oath.6F7 Next to not swearing at all, the second best thing is to keep one's oath; for, by the mere fact of swearing at all, the swearer shows that there is some suspicion of his not being trustworthy.

85 Therefore, let a man be dilatory and slow if there is any chance that by delay he may be able to avoid the necessity of taking an oath at all. But, if necessity compels him to swear, then he must consider with no superficial attention, every one of the subjects, or parts of the subject, before him. For, it is not a matter of slight importance, though from its frequency it is not regarded as it ought to be.

86 For an oath is the calling of God to give His testimony concerning the matters which are in doubt; and, it is a most impious thing to invoke God to be witness to a lie. Come now, if you please, and with your reason look into the mind of the man who is about to swear to a falsehood, and you will see that it is not tranquil, but full of disorder and confusion, accusing itself, and enduring all kinds of insolence and evil speaking.

87 For the conscience which dwells in, and never leaves the soul of each individual, not being accustomed to admit into itself any wicked thing, preserves its own nature always such as to hate evil, and to love virtue, being itself at the same time an accuser and a judge. Being roused as an accuser it blames, impeaches, and is hostile. And, again as a judge, it teaches, admonishes, and recommends the accused to change his ways, and if he be able to persuade him, he is with joy reconciled to him, but if he be not able to do so, then he wages an endless and implacable war against him, never quitting him neither by day, nor by night, but pricking him, and inflicting incurable wounds on him, until he destroys his miserable and accursed life.

In addition to a_hardin's analysis, it's important to consider the original meaning of the commandment. To take God's name didn't mean swearing (profanity), it meant swearing an oath in the name of the Lord. Swearing falsely was an extremely serious matter and continues to be one today in Semitic cultures, but to swear falsely (in vain) in the name of God was not only oathbreaking, but blasphemy as well.

Also, today many of us have taken God's name in another way. We call ourselves Christians, taking the name of Christ upon ourselves. If we claim to be Christ's disciples, and yet we don't live as one who has taken the name of Christ should, then we're in violation of this commandment. It's worth noting that the Savior's angriest and most passionate denunciation of the Jewish leaders of his day was "hypocrites!"

According to the definition of "in vain", I'd say yes, it is in vain.

in vain,
a. without effect or avail; to no purpose: to apologize in vain.
b. in an improper or irreverent manner: to take God's name in vain.

While it may not be directly insulting or condemning God in any way, I would say that "Oh my God" is not using his name in a reverent manner, specifically one that may be bringing him glory.

With regards to the habit, my wife had the same habit. After realizing that it was taking his name in vain, she was able to quickly get over it. While "Oh my gosh" has the same meaning (most likely in your case), it does remove the risk of trivializing God. Maybe once you accept that it is taking God's name in vain, you'll be able to quickly get over your habit. (I wouldn't expect anyone to get over it immediately unless you pray for God's help and he breaks it at once.)