You want to generate an OID to use with a new class or attribute you intend to add to the schema.
You have two options for generating an OID. First, you can generate a base OID off of the Microsoft OID tree. This can be done with the Oidgen.exe utility from the Resource Kit:
> oidgen Attribute Base OID: 1.2.840.1135188.8.131.5200.233.28688.28684.8.2849.956347.1967079.334190 Class Base OID: 1.2.840.1135184.108.40.20600.111.28688.28684.8.370553.291204.940269.113484
Using Oidgen is really easy, but if you want to implement schema extensions for production use, I strongly suggest you consider using an OID from your company or organization's OID branch. To determine if your company already has an assigned OID, see these sites:
If your organization does not have an assigned OID, go to your country's national registry to request one. The list of registries can be found at the following site: http://www.iso.ch/iso/en/aboutiso/isomembers/index.html.
Once you have a base OID, you can create branches from that OID however you want. For example, if you had a base OID of 220.127.116.11, you could start new class OIDs under 18.104.22.168.1 and new attributes under 22.214.171.124.2. In that case, the first class OID you would create would be 126.96.36.199.1.1 and the first attribute OID would be 188.8.131.52.2.1.
An OID is nothing more than a string of numbers separated by dots (.). OIDs were initially defined by the ITU-T in X.208 and have been used to uniquely identify a variety of things including SNMP MIB objects and LDAP schema classes and attributes. OIDs are hierarchical, and the national registries are responsible for managing and assigning OID branches.