In that case the whole OAB is being downloaded again.
We have a number of laptops in the organization, and I've noticed that it takes a long time to updated everyone's GAL on their laptops (in cached mode) when I add a new distro group or user.
The current situation, I've created a new distro group, but has yet to show up in my GAL on my laptop (cached mode of course).
This is called the Offline Address Book and is being generated on the Exchange server.
Due to various synching schedules, it can take up to 48 hours before you could actually see a change in Outlook after a modification has been made on the Exchange server or in Active Directory.
This situation however is extremely rare since the default maintenance time on Exchange is at AM and updates and downloads are usually performed during working hours.