The Lord urged Moses to have courage because Og, all his people and land, would be delivered into Israel's hand. The Lord's encouragement here could draw from Israel's past experience, where they witnessed in the conquest of Sihon that the Lord was good and dependable.
2 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.’