Having read some useful topics on the internet, I decided to try non-cached Exchange mode in Outlook. After disabling caching for my account, it still kept the OST file, even after forced deletion. Then I noticed some offline folders were being kept, like Inbox and Sent Items (right click -> Synchronization tab -> Offline folder contains...). When those were emptied, there was still something left in that OST file (<20MB). This is how you can disable offline folders altogether. Text version (for Outlook 2010):1. Menu -> File -> Info -> Account Settings -> Account Settings...
2. Select your account -> Change -> More Settings -> Navigate to Advanced tab -> Outlook Data File Settings -> Click Disable Offline Use.
Graphical version (for Outlook 2010):(screenshots are click-able) | 
