Since we can connect to Oracle without using a TNSNAMES.ORA by fully specifing de DSN entry I think the connection interface should manage this by itself.
An example of a DSN-less connectionstring:

Data Source=(DESCRIPTION=(ADDRESSLIST=(ADDRESS=(PROTOCOL=TCP)(HOST=MyHost)(PORT=MyPort)))(CONNECTDATA=(SERVER=DEDICATED)(SERVICE_NAME=MyOracleSID)));User Id=myUsername;Password=myPassword;