Yahweh is speaking to Moses as if Moses were the Israelites, so the command "do not fear" and all instances of "you" and "your" are plural. (See: [[rc://en/ta/man/jit/figs-you]])
The word "him" is a metonym for "his people." Alternate translation: "You will destroy Og and his people as you did Sihon" (See: [[rc://en/ta/man/jit/figs-synecdoche]])