This video will show how you can increase OST file size in Outlook365. By default, the OST file size can grow as large as 50 GB in office 365, but around 47.5 GB, the Outlook will stop working. There are a few ways to avoid OST getting full; one is to prevent downloading shared mailboxes in Outlook, start an Online Archive, or combine both. To increase the OST file size, we need to add a new value in the registry. And here’s how you can do it.