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):
- Menu -> File -> Info -> Account Settings -> Account Settings…
- Select your account -> Change -> More Settings -> Navigate to Advanced tab -> Outlook Data File Settings -> Click Disable Offline Use.
Graphical version (for Outlook 2010):