It’s recommended to create a new Tablespace for holding the Audit generated logs in the table SYS.AUD$  rather than leave it in the SYSTEM tablespace

However when Oracle can no longer write to the SYS.AUD$ it will refuse any further logins to the database, so space checking is important (as usual)

I created the tablespace AUDIT_DATA in the new Oracle11g Database PHKE

Then I used DBMS_AUDIT_MGMT to move the SYS.AUD$ table

SQL> begin
  2  DBMS_AUDIT_MGMT.SET_AUDIT_TRAIL_LOCATION(
  3  audit_trail_type => DBMS_AUDIT_MGMT.AUDIT_TRAIL_DB_STD,
  4  audit_trail_location_value => 'AUDIT_DATA');
  5  end;
  6  /