Considering it a Fiat or an OSCA is a difficult and debated matter: in my opinion, it's a Fiat because the cylinder block is the one casted in Turin and also has regular oil-sump and regular electric distributor. This - IMO - makes it a derivative of the Fiat 1600. Racing OSCA 1600's engines, in fact, are only similar in design, but are dry-sump and the ingnition is provided by a magneto.
However, it will be interested to know also somebody else's opionion...