Create table ro_mst
(ro_chassis_no  varchar2(17),
Ro_reg_no         varchar2(10));

Insert into ro_mst
(Ro_chassis_no,ro_reg_no)
Values
(500901,aak786);

Insert into ro_mst
(Ro_chassis_no,ro_reg_no)
Values
(500902,aak786);

Insert into ro_mst
(Ro_chassis_no,ro_reg_no)
Values
(500902,aak787);

Insert into ro_mst
(Ro_chassis_no,ro_reg_no)
Values
(500902,aak787);

Ro_chassis_no	ro_reg_no

500901		aak786
500902		aak786

SQL> SELECT * FROM ro_mst;

RO_CHASSIS_NO     RO_REG_NO
----------------- ----------
500901            aak786
500902            aak786
500902            aak787
500902            aak787

SQL>
SQL> SELECT RO_REG_NO,
  2         RO_CHASSIS_NO,
  3         Count(*) over (PARTITION BY RO_REG_NO,RO_CHASSIS_NO ) cnt_dis,
  4         Count(*) over (PARTITION BY RO_REG_NO ) cnt_reg
  5    FROM ro_mst;

RO_REG_NO  RO_CHASSIS_NO        CNT_DIS    CNT_REG
---------- ----------------- ---------- ----------
aak786     500901                     1          2
aak786     500902                     1          2
aak787     500902                     2          2
aak787     500902                     2          2

SQL>
SQL>
SQL> SELECT RO_CHASSIS_NO, RO_REG_NO FROM (
  2      SELECT RO_REG_NO,
  3             RO_CHASSIS_NO,
  4             Count(*) over (PARTITION BY RO_REG_NO,RO_CHASSIS_NO ) cnt_dis,
  5             Count(*) over (PARTITION BY RO_REG_NO ) cnt_reg
  6        FROM ro_mst)
  7  WHERE cnt_dis =1 AND cnt_reg > 1;

RO_CHASSIS_NO     RO_REG_NO
----------------- ----------
500901            aak786
500902            aak786

SQL>
SQL>

SQL> SELECT reg.RO_REG_NO,
  2         dis.RO_CHASSIS_NO
  3    FROM
  4     (SELECT RO_REG_NO,
  5             Count(*) cnt_reg
  6        FROM ro_mst
  7       GROUP BY RO_REG_NO
  8     ) reg,
  9     (SELECT RO_REG_NO,
 10             RO_CHASSIS_NO,
 11             Count(*) cnt_dis
 12        FROM ro_mst
 13       GROUP BY RO_REG_NO, RO_CHASSIS_NO
 14     ) dis
 15  WHERE dis.RO_REG_NO =  reg.RO_REG_NO
 16    AND cnt_dis = 1 AND cnt_reg > 1;

RO_REG_NO  RO_CHASSIS_NO
---------- -----------------
aak786     500901
aak786     500902

SQL>

with src as (select 500901 chassis_no,  'aak786' reg_no from dual union all 
             select 500902, 'aak786' from dual union all
             select 500902, 'aak787' from dual union all
             select 500902, 'aak787' from dual)
select * from src
where  reg_no in (select reg_no
                  from   src
                  having count(distinct chassis_no) > 1
                  group by reg_no);