WITH SYNTAX               UnboundedDirectoryString
  EQUALITY MATCHING RULE    caseIgnoreMatch
  SUBSTRINGS MATCHING RULE  caseIgnoreSubstringsMatch
  SINGLE VALUE              TRUE
  LDAP-SYNTAX               directoryString.&id
  LDAP-NAME                 {"organizationIdentifier"}
  ID                        id-at-organizationIdentifier
}

I managed to start slapd by inserting into a custom schema this:

objectidentifier        id-at-organizationIdentifier 2.5.4.97

attributeType ( id-at-organizationIdentifier
        NAME 'organizationIdentifier'
        DESC 'X520 attribute Organization Identifier'
        SUP name
        EQUALITY caseIgnoreMatch
        SUBSTR caseIgnoreSubstringsMatch SINGLE-VALUE )

But i cannot understand which objectclass can use this attribute and how to add an object using it.