Hi TitleistDBA,
It is clear that the SELECT inside the UPDATE is retrieving multiple rows.
If that is the case, then you can modify your sql as below:
update mvxcjdttst.opricl l
set l.OMFMID = (select s.odfmid
from mvxcjdttst.oprbas s,mvxcjdttst.opricl l
where l.omcono = s.odcono
and l.omitno =...