John Gill Commentary Numbers 21:34

"And Jehovah said unto Moses, Fear him not: for I have delivered him into thy hand, and all his people, and his land; and thou shalt do to him as thou didst unto Sihon king of the Amorites, who dwelt at Heshbon." — Numbers 21:34 (ASV)

And the Lord said to Moses, fear him not
Og being of a gigantic stature, and his forces numerous, might cause some fear in Moses, and in the people, and therefore the Lord encouraged them not to be afraid of him and his army:

for I have delivered him into your hand, and all his people, and his
land ;
that is, he had determined to do it, and now promised it, and it might be depended on and looked upon as if actually done:

and you shall do to him as you did to Sihon king of the Amorites,
which dwelt at Heshbon ;
slay him and his people, and take possession of his country.