All of a sudden, people start blogging about O/R mapping, thanks to Steve Eichert. Thanks Steve! :) There is however a funny thing going on in some of those blogs: they use the term ORM.
O/R mapping is something else than ORM. O/R mapping is Object-Relational mapping, ORM is Object Role Modelling. ORM is also sometimes called the successor of NIAM and has its own website at: http://www.orm.net, run by prof. T. A. Halpin, once the researcher who developed NIAM with prof. G. Nijssen and others.
Now, do they relate? Well... in an ideal world, you should be able to use an ORM model as input for the code to generate/configurate by the O/R mapper: then you have both your relational model (abstracted via the E/R model for example) and the classes targeting this relational model. Visio for Visual Studio Enterprise Architect contains an ORM tool, which lets you design your abstract relational model and eventually generate DDL from it (and / or an E/R model).
This as a FYI while you're reading the O/R mapping articles about the pro/con aspects of O/R mapping.