tente isso:
select * from mytable where instr(cell, UNISTR(<UNICODE code of your character>))>0;
exemplo:
create table mytable(
cell varchar2(100)
);
insert into mytable values('normal string');
insert into mytable values('fünny string');
commit;
select * from mytable where instr(cell, UNISTR('\00fc'))>0;
Resultado:
CELL
-----------------------------------------------------------------------------------------------
fünny string
1 row selected.
Editado:como @Wernfried Domscheit recomendou que eu mudei CHR --> UNISTR, - na verdade, isso deve funcionar com qualquer conjunto de caracteres
