There are various ways to create relationships with the Offline OData plug-in (importing functions is not yet supported). To explain the deep insets, we need more information about exactly how you are trying to do this.
You can use offline / online mode at the same time. This blog post may be helpful. It includes an example of battery life - one of which shows an example of mixed use (offline / online).
We recommend that you stay offline most of the time, and when you need to make an online call, first perform a flash / upload, resolve any data conflicts, if any, then make an online call, then make an update / download. This will ensure data consistency.