What is an OID

http://www.cubrid.org/manual/840/en/Table%20Option%20%28REUSE_OID%29

OID (Object Identifier) is an object identifier represented by physical location information such as the volume number, page number and slot number. By using such OIDs, CUBRID manages the reference relationships of objects and searches, saves or deletes them. When an OID is used, accessibility is improved because the object in the heap file can be directly accessed without referring to the table.

So it's safe to say that it's a globally unique identifier for rows, and you can use it as a shortcut to certain rows to get or set values. Also see this how-to page and the JDBC API sample:

http://www.cubrid.org/manual/830/en/JDBC%20Programming-Using%20OIDs%20and%20Collections