According to BOL, OPENDATASOURCE is used  in place of a linked server name (e.g. select * from OPENDATASOURCE(params).database.owner.table).

OPENROWSET, however, expects the tablename to be passed to it - e.g....

I'd assume the only providers that DO support the fully qualified path are SQL Server and Sybase. Oracle don't, we use OPENQUERY to interrogate Oracle 8 databases. Not sure about...