The error is received at the reader which uses the Oracle client.
You will need to try establishing a connection to the database server using the Oracle client installed on the Informatica server machine and ensure that the client connectivity works perfect.
At this point, I assume the TNS entry in the tnsnames.ora file is not configured correct or the Oracle client is not configured correct to use the correct tnsnames.ora file.
Hope this helps.
Thanks for the suggestion.
I checked the tnsnames.ora file and configured it properly and able to connect to oracle.