As a matter of fact, there are various ways of creating relationships with the Offline OData plug-in. For explaining deep inserts, more information is required about what exactly the user is trying to accomplish.
Anyways, it is possible to use both offline/online mode at the same time. Refer to this blog post. It contains several examples of offline mode. One of those examples pertains to a mixed use-case.
However, it is recommended for users to stay in the offline mode. When an online call is required, users can do a flush/upload first and resolve any data conflicts, if any. Further, they can make the online call and conduct a refresh/download.
Hence, data consistency is achieved.