How to Oracle DBLink encryption
The solution that worked for me was to use Advanced Security, with netmgr or directly edit the sqlnet.ora by adding the parameters:

SQLNET.ENCRYPTION_SERVER = REQUIRED

SQLNET.ENCRYPTION_TYPES_SERVER = (AES256)

SQLNET.ENCRYPTION_CLIENT = REQUIRED

SQLNET.ENCRYPTION_TYPES_CLIENT = (AES256)

Once this is done it is necessary to restart the listener, for access to heterogeneous data base the sslmode = require parameter must be added in odbc.ini for postgresql, the ssl access must be previously configured in the postgres server and a registry with hostssl in the file pg_hba.conf ... I have not tried for sql server yet.
Regards