John Gill Commentary Deuteronomy 3:2

"And Jehovah said unto me, Fear him not; for I have delivered him, and all his people, and his land, into thy hand; and thou shalt do unto him as thou didst unto Sihon king of the Amorites, who dwelt at Heshbon." — Deuteronomy 3:2 (ASV)

And the Lord said to me
When Og was marching with all his forces against Israel:

fear him not, etc; (See Gill on Numbers 21:34).