How to set Outlook 2003 to use .ost by policy?

I’m trying to set all Outlook’s in my company to use .ost. I want everyone to see their mail and leave it on the server. Some use .pst and when they leave and use OWA they don’t see their messages, and to fix this I would have to set one by one the settings on Outlook.

Does anyone know how to set this through GPO? Also so if any new Outlook is configured it will be set to use .ost.

Thx!