I just tried to run your sql through OEM jobs and it worked.
Here are the steps:
1. Create a job > (jobname)
2. select Destination Type i.e. (Database)
3. select Destination i.e. (Database Name)
4. In Tasks window select "Run SQL*Plus Script"
5. In Parameter window type "select sysdate from dual;" in Script Text
6. In Schedule window check radio button "Immediately"
7. Click submit & save to library
When the job completed, view job in Job History window, check Progress, highlight Completed and click Show Output.
I create the job exactly as FARRUKH described. The output generated lists the error I stated. It seems that I am creating the job correctly. The preferred credentials are for a valid database user and not and OS user, right?