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 L'Eternel me dit: Ne le crains point; car je le livre entre tes mains, lui et tout son peuple, et son pays; tu le traiteras comme tu as traité Sihon, roi des Amoréens, qui habitait à Hesbon.