If an O/R mapping tool is being used, then a new mapping section will have to be added to the mapping file. And, in both cases, a new database table (possibly more than one) will have to be created. In short, adding a new persistent class goes beyond simply adding the class; an entourage of additional infrastructure must be assembled.
In a true ODBMS, none of the above work is necessary. For example, using the open-source object database, db4o, the programmer simply begins storing the new objects, and the database is automatically adjusted to accept them. The db4o engine invisibly discovers the new class structure (via reflection) and performs all the behind-the-scenes tasks to store the new object type. There is no administrative work, no new tables need to be added, and no mapping entries must be written.