Check table which not having primary key in Oracle Database

select  OWNER,
	TABLE_NAME
from    dba_tables dt
where   not exists (
        select  'TRUE'
        from    dba_constraints dc
        where   dc.TABLE_NAME = dt.TABLE_NAME
        and     dc.CONSTRAINT_TYPE='P')
and 	OWNER not in ('SYS','SYSTEM')
order	by OWNER, TABLE_NAME

For check specific schema or owner use the following query:

select  OWNER,
	TABLE_NAME
from    dba_tables dt
where   not exists (
        select  'TRUE'
        from    dba_constraints dc
        where   dc.TABLE_NAME = dt.TABLE_NAME
        and     dc.CONSTRAINT_TYPE='P')
and 	OWNER = 'HR'
order	by OWNER, TABLE_NAME