As part of developing an application, designing system configuration, or due to other requirements, you will occasionally have to add additional attributes or even define a new objectClass.

In order to do that you will need to register these new schema elements with the Internet Assigned Numbers Authority (IANA.) The IANA operates an online service you can use to get an Object ID (both attributes and object classes are considered "objects" needing a unique ID).

IANA keeps track of private enterprise numbers (PENs) of private organizations. If your organization does not have a PEN allocated yet, please go to https://pen.iana.org/pen/PenApplication.page. IANA will assign a PEN which typically looks something like 1.3.6.1.4.1.XXXXX where XXXXX is the assigned number for your organization.

Once you have the base OID, you can create branches (sub-OIDs) for your specific object classes and attributes. A common practice is to use .1 for attributes and .2 for object classes, but you can define your own structure. 

For example, if your assigned base OID is 1.3.6.1.4.1.77777, you might create branches like: 
1.3.6.1.4.1.77777.1.1 (where 1.3.6.1.4.1.77777.1. is a prefix you use for all attributes)
1.3.6.1.4.1.77777.2.1 (where 1.3.6.1.4.1.77777.2. is a prefix you use for all object classes)

It is never appropriate to make up an OID or use someone else's. They are free and easy and your OID will be unique. That means that if you ever have to use back-ldap to connect to another directory or back-meta to create a virtual/meta directory, all the OIDs will be unique.