Serialization provided by the most of Language lets you flaten the Object attributes and thus storing easily them into the RDBMS and similarly retrieving objects is not a big issue. The wide and solid foundation still lacks which hinders the use of OODBMS to be implemented.
I currently thinking of doing this as my Master Thesis project to provide a general framework for OODBMS that supports almost all the components which is commonly used in now a day RDBMS thus providing a non-linear structured DBMS. While studying I came across a project called db4o which is an approach (implemented) of using OODBMS for Java and .net only, so this could be another reason of lack of generality for all types of platforms and languages.