select listagg(letter) 
    within group (order by nlssort(letter, 'nls_sort=german') 
from
(
select regexp_substr('gfedcbaGFEDCBA', '.', level) as letter from dual
connect by regexp_substr('gfedcbaGFEDCBA', '.', level) is not null
);

with
t(a, b) as
(select 'hello', 'alloha' from dual
union all select 'hello', 'elloh' from dual
union all select 'hello', 'hell' from dual
union all select 'hello', 'helo' from dual
union all select 'hello', 'elloha' from dual)
select * from t where
xmlquery('for $i in string-to-codepoints(.) order by $i return $i' passing lower(a) returning content).getstringval() =
xmlquery('for $i in string-to-codepoints(.) order by $i return $i' passing lower(b) returning content).getstringval()
/