OAB was designed for the user who work on cach mode so that they can have use the address book when they are not connected to the exchange server or in other words when they are working offline.
so if the a user is connect to the exchange in cache mode in outlook that he will be only looking at the OAB.
similarly when a user is connected to the exchange server in online mode he will be looking at the GAL and not the OAB.