In preparation for an essay in which I would like to explain the meaning of some of the midrashim about Og, I would like to go through what the Torah has to say about this "giant". My method will be to go through the verses and paraphrase each mini-unit/paragraph of text. Along the way, I will try to make note of some the more interesting elements that could easily be missed in a cursory reading.
Disclaimer: please consider this as a rough first reading. Any critiques or suggestions will be appreciated.
It seems, as with many of the events in the Torah, that it all goes back to the B'rit Bein HaBetarim. At the conclusion of that B'rit, God assured Avraham that his descendants would inherit the land of Canaan. However, this inheritance had to be delayed until the "sin of the Emori is complete" (not to exclude the other reasons it was delayed).
וְדוֹר רְבִיעִי, יָשׁוּבוּ הֵנָּה: כִּי לֹא-שָׁלֵם עֲוֹן הָאֱמֹרִי, עַד-הֵנָּה. (בר' טו.טז)
Finally, in the twenty-first chapter of B'midbar (vv. 21-35) the day seems to have arrived:
כא וַיִּשְׁלַח יִשְׂרָאֵל מַלְאָכִים, אֶל-סִיחֹן מֶלֶךְ-הָאֱמֹרִי לֵאמֹר. כב אֶעְבְּרָה בְאַרְצֶךָ, לֹא נִטֶּה בְּשָׂדֶה וּבְכֶרֶם--לֹא נִשְׁתֶּה, מֵי בְאֵר: בְּדֶרֶךְ הַמֶּלֶךְ נֵלֵךְ, עַד אֲשֶׁר-נַעֲבֹר גְּבֻלֶךָ.
Israel sent messengers (note the similarity to B'reishit 32:4: "וַיִּשְׁלַח יַעֲקֹב מַלְאָכִים לְפָנָיו, אֶל-עֵשָׂו "אָחִיו, אַרְצָה שֵׂעִיר, שְׂדֵה אֱדוֹם) to Sichon king of the Emori requesting passage through his land - giving him their word that they would stay on the King's Highway and not tread on anyone's field, or vineyard, or drink from any wells. However, he did not consent:
כג וְלֹא-נָתַן סִיחֹן אֶת-יִשְׂרָאֵל, עֲבֹר בִּגְבֻלוֹ, וַיֶּאֱסֹף סִיחֹן אֶת-כָּל-עַמּוֹ וַיֵּצֵא לִקְרַאת יִשְׂרָאֵל הַמִּדְבָּרָה, וַיָּבֹא יָהְצָה; וַיִּלָּחֶם, בְּיִשְׂרָאֵל.
Even more shockingly he seemed to perceive Israel as a threat and gathered "his entire nation" to the desert and waged war against them! However, he was not triumphant:
כד וַיַּכֵּהוּ יִשְׂרָאֵל, לְפִי-חָרֶב; וַיִּירַשׁ אֶת-אַרְצוֹ מֵאַרְנֹן, עַד-יַבֹּק עַד-בְּנֵי עַמּוֹן--כִּי עַז, גְּבוּל בְּנֵי עַמּוֹן. כה וַיִּקַּח, יִשְׂרָאֵל, אֵת כָּל-הֶעָרִים, הָאֵלֶּה; וַיֵּשֶׁב יִשְׂרָאֵל בְּכָל-עָרֵי הָאֱמֹרִי, בְּחֶשְׁבּוֹן וּבְכָל-בְּנֹתֶיהָ.
Israel soundly defeated them and took possession of his entire land to the border of Amon. They also took hold of the cities and settled in them - this included Cheshbon and all its provinces. In high poetic style the Torah proceeds to explain how Cheshbon came to belong to Sichon:
כו כִּי חֶשְׁבּוֹן--עִיר סִיחֹן מֶלֶךְ הָאֱמֹרִי, הִוא; וְהוּא נִלְחַם, בְּמֶלֶךְ מוֹאָב הָרִאשׁוֹן, וַיִּקַּח אֶת-כָּל-אַרְצוֹ מִיָּדוֹ, עַד-אַרְנֹן. כז עַל-כֵּן יֹאמְרוּ הַמֹּשְׁלִים, בֹּאוּ חֶשְׁבּוֹן; תִּבָּנֶה וְתִכּוֹנֵן, עִיר סִיחוֹן. כח כִּי-אֵשׁ יָצְאָה מֵחֶשְׁבּוֹן, לֶהָבָה מִקִּרְיַת סִיחֹן: אָכְלָה עָר מוֹאָב, בַּעֲלֵי בָּמוֹת אַרְנֹן. כט אוֹי-לְךָ מוֹאָב, אָבַדְתָּ עַם-כְּמוֹשׁ; נָתַן בָּנָיו פְּלֵיטִם וּבְנֹתָיו בַּשְּׁבִית, לְמֶלֶךְ אֱמֹרִי סִיחוֹן. ל וַנִּירָם אָבַד חֶשְׁבּוֹן, עַד-דִּיבֹן; וַנַּשִּׁים עַד-נֹפַח, אֲשֶׁר עַד-מֵידְבָא.
After this poetic interlude the Torah repeats that Israel settled in the land of the Emori (I consider this the beginning of the second half of the story.):
לא וַיֵּשֶׁב, יִשְׂרָאֵל, בְּאֶרֶץ, הָאֱמֹרִי.
Now, the Torah shifts its focus from Israel (note that only Israel has been mentioned up to this point) to Moshe. Moshe sent spies - most likely they were spies, however, the Torah leaves out the indirect-object leaving us to fill it in based on context (I can only wonder why the Torah would not want to mention spies (m'raglim) - hmm) - on a reconnaissance mission to gather information on Ya'zeir and "they" (most likely Israel - again, the Torah is somewhat ambiguous) captured all of its provinces and uprooted the Emori that were there:
Next, "they" turned and went up by way of Bashan:
לג וַיִּפְנוּ, וַיַּעֲלוּ, דֶּרֶךְ, הַבָּשָׁן; וַיֵּצֵא עוֹג מֶלֶךְ-הַבָּשָׁן לִקְרָאתָם הוּא וְכָל-עַמּוֹ, לַמִּלְחָמָה--אֶדְרֶעִי.
Og, the king of Bashan came out to "greet them" - he, and his entire nation for war. (Note how the Torah emphasises Og's presence at the battle by singling him out with the word "הוא (he)", "he, and his entire nation" went out to battle.)
לד וַיֹּאמֶר יְהוָה אֶל-מֹשֶׁה, אַל-תִּירָא אֹתוֹ--כִּי בְיָדְךָ נָתַתִּי אֹתוֹ וְאֶת-כָּל-עַמּוֹ, וְאֶת-אַרְצוֹ; וְעָשִׂיתָ לּוֹ--כַּאֲשֶׁר עָשִׂיתָ לְסִיחֹן מֶלֶךְ הָאֱמֹרִי, אֲשֶׁר יוֹשֵׁב בְּחֶשְׁבּוֹן. לה וַיַּכּוּ אֹתוֹ וְאֶת-בָּנָיו וְאֶת-כָּל-עַמּוֹ, עַד-בִּלְתִּי הִשְׁאִיר-לוֹ שָׂרִיד; וַיִּירְשׁוּ, אֶת-אַרְצוֹ.
Before the battle, God instructed Moshe to not be afraid of him (meaning Og) because He had delivered him (Og) and his entire nation and his land into his (Moshe's) hand. God assured Moshe that he would do to him (Og) what he did to Sichon, king of the Emori, who dwells in Cheshbon (The Torah's use of the "present tense" is most probably explained by the fact that, technically, there are no tenses in BH. As in bonei Yerushalayim, the "present tense" can also be used as a noun - this is called a participle, meaning, it participates, or partakes, in the nature of both a noun (as an adjective) and a verb. Check out this article for a nice, simple explanation of tenses in BH). Indeed, Moshe struck Og and his sons (who we hear about, now, for the first time) and his nation to the point that no one remained. And they took possession of Og's land.