The OST file as most people will know is the client side “local cache” that the Outlook client creates. Also caused cached mode Outlook, by many people. A question was raised the other day which goes along the lines of :
If I have 5000 items in here, and they all get archived and turned into shortcuts, will the OST shrink?
At first I thought it wouldn’t shrink, but I was wrong. Per the Microsoft article : http://support.microsoft.com/kb/289987/en-us, the OST will shrink automatically, when idle CPU is detected. I tested this as follows :
I just inserted 5000 random messages in to my mailbox.. in online mode.
I then setup cached mode.
The OST file is now : 49.85 Mb
I then deleted ALL the items in the folder (shift deleted them), and waited a while.
The OST file is now : 54.8 Mb
I then purged the deleted items (by going into Recover Deleted Items, and clearing the items), and waited a couple of hours.
Now the OST is just 1.1 Mb
So the Microsoft article is proven to be correct.