Numbers 21:34

Do not be afraid of him. Moshe was afraid to do battle, for perhaps, the merit of Avrohom, would stand for him [Og] as it is said, “the one who had escaped came.” This was Og, who escaped, of the Rephaim whom Kedorlaomer and his cohorts smote in Ashteros Karnayim, as it is said, “for only Og, king of Bashan, was left of the remaining Rephaim.”

And it was, when Mosheh saw Og, he trembled before him, stricken with fear: but he (soon) answered and said, This is Og the Wicked, who taunted Abraham our father and Sarah, saying: You are like trees planted by the water channels, but bring forth no fruit: therefore hath the Holy One, blessed be He, spared him to live through generations, that he might see the great multitude of their children, and be delivered into our hands. Then spake the Lord unto Mosheh: Fear him not, for I have delivered him into thy hand, and all his people and country; and thou shalt do to him as thou hast done to Sihon, king of the Amorites, who dwelt in Heshbon.

FEAR HIM [Og] NOT. “For Moses was afraid [to wage war against Og] in case the merit of Abraham [whom Og had helped] would stand him in stead, for it is said about him, And there came ‘hapalit’ (one that had escaped) and told Abram the Hebrew, this being Og who had escaped from the Rephaim whom Amraphel and his allies had defeated at Ashteroth-karnaim.” This is Rashi’s language, based on the words of our Rabbis. 25; Bamidbar Rabbah 19:19. Here too the Rabbis were induced to make this comment because they knew that Moses our teacher would not have been afraid of an arm of flesh, for with him was the Eternal our G-d, and all the nations are as nothing before Him; they are accounted by Him as things of nought, and vanity. [Furthermore], it was he [Moses himself] who admonished Israel: fear not, nor be dismayed on account of them, and who criticized them for being afraid of them, as it is said in connection with the [affair of the] spies, Then I said unto you: ‘Dread not, neither be afraid of them. The Eternal your G-d, Who goeth before you, He shall fight for you.’, 1:29-30. Therefore [we must say] that Moses was afraid [of Og] because he knew of his merit [in having helped Abraham].
In accordance with the plain meaning of Scripture, Moses had not intended now to give Israel the land of Sihon and Og as an inheritance, as I have explained. But Sihon had gone out against Israel into the wilderness,, Verse 23. and had fought them against their will [in spite of their desire for peace]. Then G-d informed [Moses], Behold, I have begun to deliver up Sihon and his land before thee; begin to possess his land, for here began the conquest of the seven nations [who inhabited the Land of Israel]. But Og [nonetheless] mobilized all his forces at Edrei, which was a city at the end of his border, and Israel could have turned away from him just as they had turned away from Esau [i.e., Edom]. Therefore G-d told Moses, “Fear him not; go to him and contend with him in battle, for I have delivered him into thy hand.” And in Bamidbar Sinai Rabbah I have seen [the following text] “Why was Moses afraid [of Og]? He said: Perhaps Israel committed a trespass in the war against Sihon, [by taking of the spoil for themselves], or maybe they have become defiled by sins.” This is a completely homiletic exposition. And indeed all righteous people have this fear [that they might have sinned in error], and this [fear] likewise occurred [in the case] of Joshua [about whom it says that G-d told him, Fear not, neither be thou dismayed].
Balak

ויאמר ה' אל משה אל תירא אותו, Hashem said to Moses: “do not be afraid of him!” Rashi explains that there was a reason why G–d had to say this to Moses concerning Og, although He had not said this to him before joining battle with Sichon. Moses was afraid that Og had acquired a great merit by warning Avraham at the time that his nephew Lot had been taken prisoner by a King who had thought that he was Avraham, as he had meant to kill or capture Avraham. Compare Genesis 14,13, and the explanation in the Talmud tractate Niddah folio 61 on that verse. If you were to say that according to that statement in the Talmud, Og had actually meant to kill Avraham so that he could marry Sarah, i.e. hardly an intention qualifying for additional merit, the fact that he was instrumental in saving Avraham at that time and in saving Lot by Avraham coming to his rescue, was enough to make Moses fear him. [After all this Og was at least 500 years old if he had been a somebody already at that time. Ed.] The Talmud in tractate Sanhedrin folio 105 tells us that the 42 burnt offerings offered by Balak to Hashem at the direction of Bileam, was enough for him to be rewarded that Ruth the Moabite became descended from him. Balak had also intended only to cause Israel harm at that time, and nonetheless the sages of the Talmud felt that G–d had to reward him. Some sages understand the fact that Og was described –though indirectly- as a fugitive, in Genesis, this means that he had escaped not Nimrod, but that he had escaped the deluge and had managed to survive it somehow. This would be good reason for Moses to feel that he had G–d’s protection even at his time. He is also supposed to have been a brother of Sichon, giving him good reason to attack Moses who had killed his brother. Our author refers to what he had explained in Parshat Noach in the name of Rabbi Yehudah hachassid who had pointed out that the numerical value of the words אך נח in the line: וישאר אך נח, “only Noach survived,” is identical to the numerical value of the word עוג, i.e. 79. (Genesis 7,23) There is also a statement by Rabi Yechiel bar Yoseph, according to whom Og had been born before the onset of the deluge, his mother having been pregnant with him at the time, and having married one of Noach’s sons, her pregnancy having been caused by one of the people whom t*he Torah had described as בני האלוהים, usually understood as “fallen angels.”