The adult male is not likely to be confused with any other duck. The very dark female is somewhat similar to female scoters but note the smaller size, smaller bill, rounder head, and different face pattern. The female Bufflehead is also small but only has one white spot on the head and a white speculum. In flight, the female Oldquaw also has dark wings but has more white in the head and body.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.