There is only one file type that uses the .OLM extension:
Mac Outlook Database File
Database file used by Microsoft Outlook for Mac; used as the software's internal format for storing email messages, contacts, journals, calendar data, and other user application data.
OLM files are used only by Mac Outlook and can't be opened by the Windows version of Outlook because the Windows version uses .PST files rather than the OLM format. To open OLM files in Windows, you can first convert the OLM file to the PST format with Aryson OLM to PST Converter and then open it with Outlook.
NOTE: The Stellar Phoenix OLM Repair tool is designed to repair corrupt Mac Outlook OLM files. The program can help recover emails, calendars, address books, attachments, notes, and tasks.
Updated: July 16, 2018