On a Debian installation I have just installed the OpenLDAP server `slapd` with:

 ~$ sudo apt install slapd ldap-utils
 ~$ sudo dpkg-reconfigure slapd

On its setup with default options I was prompted to give an organisation name. I used *home*, so I get

 dn: o=home,dc=hoeft-online,dc=de

Now I try to add an organisational unit (**ou**) to the organisation (**o**) without success with:

 ~$ cat add.ldif
 dn: ou=posix,o=home,dc=hoeft-online,dc=de
 objectClass: organizationalUnit
 ou: posix

 ~$ ldapadd -xWD cn=admin,dc=hoeft-online,dc=de -f add.ldif
 Enter LDAP Password:
 adding new entry "ou=posix,o=home,dc=hoeft-online,dc=de"
 ldap_add: No such object (32)
 matched DN: dc=hoeft-online,dc=de

Adding an **ou** to the domainComponent (**dc**) works:

 ~$ cat add.ldif
 dn: ou=posix,dc=hoeft-online,dc=de
 objectClass: organizationalUnit
 ou: posix

 ~$ ldapadd -xWD cn=admin,dc=hoeft-online,dc=de -f add.ldif
 Enter LDAP Password:
 adding new entry "ou=posix,dc=hoeft-online,dc=de"

What I'm missing here? Isn't it possible to add an organizationalUnit (**ou**) to an organisation (**o**)? If not, why? Where is it defined?