I discovered the answer in the Oracle Identity Management docs.

To construct multiple domains, it is necessary to use "Create Like" in the Oracle Directory Manager to create the "dc" parts.

For example, use "Create Like" to copy dc=de to dc=fr. Then use "Create Like" to make a copy of dc=mycompany from the dc=de tree to the dc=fr tree.

If the changes do not show up immediately, exit the directory manager and immediately re-enter.

After that it is necessary to use the oidca tool and the CREATECTX option to add the OracleContext.

For example

oidca mode=CREATECTX -silent oidhost=OID Server hostname nonsslport=389 sslport=636 dn=cn=orcladmin pwd=password contextdn=cn=OracleContext,dc=mycompany,dc=fr

That will create

cn=OracleContext,dc=mycompany,dc=fr

IMHO, the documentation and lack of concrete examples in the Oracle Docs is strange considering that OID seems to be the direction for network resolution.