If you are facing this same error “Could not load file or assembly ‘Oracle.DataAccess’ or one of its dependencies. An attempt was made to load a program with an incorrect format.” using the Oracle.DataAccess assembly in my case , and only if it happens using the iis version of your WCF service, but running it directly from Visual Studio works , then you problem is on the Application Pool on your IIS

Go to the Application Pool of your application on the IIS:

Select Advanced Settings:

And change Enable 32-bit Applications to TRUE

And finally make an IIS Reset

This option solve my problem.