Some of the common Oracle error codes:

ORA-12500: TNS:listener failed to start a dedicated server process
ORA-12541: TNS:no listener
ORA-12514: TNS:listener does not currently know of service requested
ORA-12154: TNS:could not resolve service name
ORA-12535: TNS:operation timed out
ORA-01017: invalid username/password; logon denied

ORA-00001 ORA-00054 ORA-00257 ORA-00600 ORA-00604 ORA-00904 ORA-01008 ORA-01017 ORA-01401 ORA-01422 ORA-01427 ORA-01460 ORA-01652 ORA-01653 ORA-01654 ORA-01830 ORA-01841 ORA-01843 ORA-01858 ORA-01861 ORA-02291 ORA-03135 ORA-03505 ORA-04068 ORA-12154 ORA-12170 ORA-12500 ORA-12504 ORA-12505 ORA-12514 ORA-12518 ORA-12520 ORA-12528 ORA-12535 ORA-12541 ORA-12545 ORA-12560 ORA-12705 ORA-12899 ORA-17002 ORA-17003 ORA-17041 ORA-20001 ORA-27101 ORA-28009 ORA-30036 ORA-38301 ORA-40508

Latest Forum questions:

ORA-12154: Cannot connect locally
ORA-12514: Problem while connecting to the database
ORA-12541: Is listener running?
ORA-12541: Oracle Listener
ORA-01017: Different tnsnames.ora