Opush is a free ActiveSync server connected to OBM. It permits to synchronize your obm account with the native applications of your device.
So you have no application to install on your device and you will enjoy every capabilities offered by your favorite device : iPhone, iPad, Androids, Windows Phone, Windows Mobile ...
What can be synchronized with your Opush ?
Every ActiveSync client should be capable to be synchronized with OBM using Opush.
To know what kind of phones are officialy supported, please take a look here.
What Opush is able to synchronize ?
Opush will manage in both way, from your device to OBM and from OBM to your device !