Linking Oracle 10g XE with SQL server using ODBC
Use the parameter NAMES.DEFAULT_DOMAIN to set the domain from which the client most often looks up names resolution requests. When this parameter is set, the default domain name is automatically appended to any unqualified net service name or service name.

For example, if the default domain is set to us.acme.com, the connect string CONNECT scott/tiger@sales gets searched as sales.us.acme.com. If the connect string includes the domain extension, such as CONNECT scott/tiger@sales.acme.com, the domain is not appended.