Object id's have some limitations. The global nature of OID numbering means that most OID's in a table are not sequential. For example, if you insert a customer today, and another one tomorrow, the two customers may get OID's that differ by thousands, depending on how many other INSERTs occured between the creation of the two customers. If the OID is not visible to users, this is not a problem. However, if users see and enter these numbers frequently, it may seem strange that their customer id's are not sequential or have large gaps in numbering. OIDs are best used in situations where sequential numbering is not important.