Usunąłem pierwszy distinct skoro już jesteś group by wszystkie pola w Twoim Select zapytanie i zamieniłem case when z select zapytanie:
select ds.catnr,ds.planqty, ds.ordnr, ds.posnr, ds.segnr,
listagg((select distinct max(li1.paco) from leos_item li1 where li.av_part_no = li1.av_part_no and li.paco is not null), ', ') within group (order by pd.part_no) inq_no
from oes_delsegview ds, oes_address ad, oes_opos op, oes_nrbom nr, scm_prodtyp sp, leos_item li, part_description pd
where ds.delnr = ad.key
and ad.adr = ds.deladr
and ds.pos_o_status not in ('9', 'D')
and ds.pos_c_status not in ('9', 'D')
and ds.seg_o_status not in ('9', 'D')
and ds.seg_c_status not in ('9', 'D')
and ds.cunr in ('W31170','W31172')
and ds.pos_type != 'RC'
and ds.ordnr = op.ordnr
and ds.posnr = op.posnr
and ds.catnr = pd.catnr
and ds.prodtyp = pd.prodtyp
and ds.packtyp = pd.packtyp
and ds.catnr = nr.p_catnr (+)
and ds.prodtyp = nr.p_prodtyp (+)
and ds.packtyp = nr.p_packtyp (+)
and nr.c_prodtyp = sp.prodtyp (+)
and sp.prodgrp (+) = 'COMP'
and substr(nr.c_prodtyp,1,2) not in ('MT','LF')
and nr.c_catnr = li.catnr (+)
and nr.c_prodtyp = li.prodtyp (+)
and nr.c_packtyp = li.packtyp (+)
and pd.catnr = '9780007938797'
group by ds.catnr,ds.planqty, ds.ordnr, ds.posnr, ds.segnr
