Display all schemas in Oracle Database

select distinct owner schema_name

from dba_segments

where owner in(select username from dba_users where default_tablespace not in ('SYSTEM','SYSAUX'))

order by owner;

Or

select distinct owner schema_name

from dba_segments

where owner not in ('SYSTEM', 'XDB', 'SYS', 'TSMSYS', 'MDSYS', 'EXFSYS', 'WMSYS', 'ORDSYS', 'OUTLN', 'DBSNMP')

order by owner;