OLM is a data file that is supported by Outlook for Mac. It can store all kinds of mailbox data such as emails, contacts, calendars, notes, tasks, journals, etc. A single OLM file can store a complete Mac Outlook mailbox.
The first time, it was introduced with Mac Outlook 2011 as its master repository file format to store local data of Outlook. Now, it is supported by all versions of Mac Outlook.
Outlook for Mac does not create OLM file by default. There are various export options offered by Mac Outlook to create it. Although there is no maximum size limit set for Mac OLM file but its size keep under 2GB to avoid corruption and data loss.
How to Open OLM File?
OLM file extension is only supported by Mac Outlook. You can view and read this file in Outlook for Mac. It cannot be opened in the Windows system. To open in a Windows-based system, you need to convert OLM to PST, PDF, and other formats. You can also open, read, and preview OLM data in Windows by third-party OLM Viewer.