https://bugs.openldap.org/show_bug.cgi?id=10411
--- Comment #5 from elecharny@apache.org --- Regardless of how slapadd works, I still think that when modifying an entry that does not have a structuralObjectClas opAttr should work: it's not really a problem to detect that the entry does not have this OpAttr, and to compute one on the fly while processing the Modify operation.
But that would be another bugzilla issue.