Argento only helped writing the story for OUTW togther with Bertolucci and Leone. The screenplay was written by Leone and Sergio Donati.
Donati had complained about Argento for claiming contributions he hasn't done.
Robert Hossein denies any work of Argento on CWC. He isn't credited on the french version. He is credited on the italian version, maybe because he really did work on CWC, maybe only to fulfill some of these obscure italian co-production foundations.
He is credited for 5 Man Army and Today It's Me (together with director Cervi).