The Torah states that God bolstered Moses’ spirit in preparation for the war with Og king of Bashan. The Sages explain why Moses was concerned specifically about this confrontation.

What is the difference between Og and Sihon, that Moses feared Og but did not fear Sihon?… Moses said: Perhaps the merit of assisting Abraham our patriarch will help Og, as it is stated (Genesis 14:13): “The survivor came and told Abram the Hebrew” [that Lot, Abraham’s nephew, had been captured in battle]. And Rabbi Yohanan said: This survivor is Og, who survived the generation of the flood. (Nidda 61a)