Following, a useful sql request listing system tables sizes in descending order. It helps to detect which table data could be purged to save more disk space :

select owner, segment_name, segment_type, bytes / 1024 / 1024 "size"
  from dba_segments
 where tablespace_name = 'SYSTEM'
 order by "size" desc;

Audit table SYS.AUD$ could be the cause of full disk space. Administrator must control the growth and size of the audit trail.