Deu 3:1-8 “Then we turned and went up the way to Bashan. And Og the king of Bashan came out against us, he and all his people, to battle at Edrei. But the LORD said to me, ‘Do not fear him, for I have given him and all his people and his land into your hand. And you shall do to him as you did to Sihon the king of the Amorites, who lived at Heshbon.’ So the LORD our God gave into our hand Og also, the king of Bashan, and all his people, and we struck him down until he had no survivor left. And we took all his cities at that time—there was not a city that we did not take from them—sixty cities, the whole region of Argob, the kingdom of Og in Bashan. All these were cities fortified with high walls, gates, and bars, besides very many unwalled villages. And we devoted them to destruction, as we did to Sihon the king of Heshbon, devoting to destruction every city, men, women, and children. But all the livestock and the spoil of the cities we took as our plunder. So we took the land at that time out of the hand of the two kings of the Amorites who were beyond the Jordan, from the Valley of the Arnon to Mount Hermon
Deu 3:21-22 And I commanded Joshua at that time, ‘Your eyes have seen all that the LORD your God has done to these two kings. So will the LORD do to all the kingdoms into which you are crossing. You shall not fear them, for it is the LORD your God who fights for you.’
Israel continued closer to the Promised Land, moving westward towards the Jordan River, they passed through the land of Og, king of Bashan. This brought Israel even more territory to occupy on the east side of the Jordan River, and it showed them that they could, through the power of God, overcome the mighty enemies they would confront on the west side of the Jordan River. The repeated references to the rephaim in these first three chapters shows that Israel, when trusting in God, was well able to defeat this race of fearsome warriors. It also shows that their fear of these men back in Numbers 13, where they first refused to go into the Promised Land, was unfounded. Their excuses are shown to be weaker in light of the next generation’s victories. The two-and-one-half tribes decided that this land on the east side of the Jordan River was good enough for them, and the LORD allowed it – if they would fulfill the obligations mentioned in the following verses. (Guzik)
Og was very powerful, but he did not take warning by the ruin of Sihon, and desire conditions of peace. He trusted his own strength, and so was hardened to his destruction. Those not awakened by the judgments of God on others, ripen for the like judgments on themselves. Moses encouraged Joshua, who was to succeed him. Thus the aged and experienced in the service of God, should do all they can to strengthen the hands of those who are young, and setting out in religion. Consider what God has done, what God has promised. If God be for us, who can be against us, so as to prevail? (Henry)