Directory servers like OpenDJ come with schema definitions for a wide range of standard object classes and attribute types. This is because directories are designed to be shared by many applications. Directories use unique, typically IANA-registered object identifiers (OID) to avoid object class and attribute type name clashes. The overall goal is Internet-wide interoperability.
You therefore should reuse schema definitions that already exist whenever you reasonably can. Reuse them as is. Do not try to redefine existing schema definitions.
If you must add schema definitions for your application, extend existing object classes with AUXILIARY classes of your own. Take care to name your definitions such that they do not clash with other names.
When you have defined schema required for your application, work with the directory administrator to have your definitions added to the directory service. Directory servers like OpenDJ let directory administrators update schema definitions over LDAP, so there is not generally a need to interrupt the service to add your application. Directory administrators can however have other reasons why they hesitate to add your schema definitions. Coming to the discussion prepared with good schema definitions, explanations of why they should be added, and evident regard for interoperability makes it easier for the directory administrator to grant your request.