I think this is simpler than the replies assume (or I may be missing
something, which is not uncommon). I don't think tables joined through a
left outer join is supposed to be in the main where clause. Basically, your
query ought to be written like

select M.DESCRIZIONE from MACCHINE m
left outer join DEPOSITI d on d.oid=m.oid_deposito and d.oid=13055
left outer join CLASSI_MACCHINE c on c.oid=m.oid_classe_macchina

Still, it doesn't make sense to do an outer join to a table without
including any fields from the table in the result set.

HTH,
Set