Why did Hashem find it necessary to reassure Moshe before the battle with Og more than before the battle with Sichon?
Rashi: Refer to Bamidbar, 21:34:1:1-4. 1
Oznayim la'Torah #2 (citing the Yalkut Shim'oni): Moshe figured that since Og had lived to the age of five hundred, he must have merits 2 - of which he was afraid. 3
Oznayim la'Torah: According to the Yalkut Shim'oni he was the strongest man who ever lived.
Refer to Bamidbar, 21:34:1:2 & 21:34:2:1*.
See Oznayim la'Torah DH 'Al Tiyra oso; #2.
Having written in Pasuk 31 "Re'ei Hachilosi Teis lefanecha es Sichon", why did Hashem need to state here "ki be'Yadcha Nasati oso"?
Oznayim la'Torah: It was a hint that He will give Og into the hands of Moshe personally - that Moshe will be the one to kill Og.