An administrator had a question about having Outlook in online mode, yet an OST file was created.
I have Outlook Cached Mode disabled in Group Policy so that no one can turn it on and yet OST files get created. When I open Outlook, the status bar says it's online, but after about 2 seconds you will get the Send & Receive status bar for the account and the OST file in the %localappdata%\microsoft\outlook folder grows.
Outlook has one Online mode but two offline modes: Exchange Cached mode and Offline Folders. Although Outlook 2013 doesn't use true offline folders, only cached mode, you need to disable offline folders along with cached mode to prevent it and older versions of Outlook from creating an OST file for offline use.
You can disable offline use using group policy or by setting a registry value.
In the Group Policy editor, go to Microsoft Outlook/Account Settings/Exchange and set Do not allow an OST file to be created to Enable. In the Cached Exchange mode folder, set Use Cached Exchange mode for new and existing Outlook profiles to disabled.
These policies will set the following registry values
NoOst value specifies whether or not an OST is created at startup. Setting the NoOst value to 2 will prevent the use of OST files. A value of 3 will allow OST files for cached mode but won't create one when online mode is used.
|0||An OST is created by default and used for calendar caching. You can enable offline access and use the offline store. This is the default if the NoOST entry is missing.|
|1||An OST is set up by default and used for calendar caching. Users cannot enable offline access or use the offline store. The options in the Exchange Server properties are not available.|
|2||An OST is not set up by default and you cannot enable offline access or use an offline store. The options in the Microsoft Exchange Server properties are not available.|
|3||An OST is not set up for Online mode. Users can enable offline access and use the offline store.|