OLM files are outlook files that run on Mac Outlook. Olm files contain data related to mailbox like emails, calendars, tasks, journals. 

Silky Terrier Dog Breed Playing Aro... Silky Terrier Dog Breed Playing Around
The only downside is that the OLM file cannot be opened on windows outlook application.  Silky Terrier Dog Breed Playing Around

OLM files can be opened through 

• Microsoft outlook 365

• Stellar Repair 
• Aryson OLM converter