Avoid already existing objects when importing into Oracle
Exclude the schemas with the exclude parameter. E.g:

EXCLUDE=SCHEMA:\"IN\(\'SYS\',\'SYSTEM\',\'OUTLN\',\'ORDDATA\',\'OLAPSYS\',\'MDDATA\',\'SPATIAL_WFS_ADMIN_USR\',\'ANONYMOUS\',\'SPATIAL_CSW_ADMIN_USR\',\'ANONYMOUS\',\'FLOWS_FILES\',\'APEX_PUBLIC_USER\',\'APEX_030200\',\'OWBSYS\',\'OWBSYS_AUDIT\',\'SCOTT\',\'SYSMAN\',\'MGMT_VIEW\'\)\"


The list of schemas depends on the database version and options installed.

In a parameterfile, you can use the above without \ (escaping special characters).