Commentary for Deuteronomy 3:10

ערי ממלכת עוג בבשן....כי רק עוג מלך הבשן נשאר, the reason why he ruled in that district was because he was the only one remaining from the Refaim of old. Ever since the war between Amrafel and his allies who defeated the Refaim on their way to Sodom at Ashterot Karnayim (Genesis 14,5), there were numerous individual Refaim remaining. These defeated the Ammonites as mentioned in 2,21. During that war against the Ammonites only Og survived. Seeing that he was a famous hero, equaling the combined power of all the other Refaim, he was the one who ruled.