An OLM file is always generated by the Outlook for Mac application. It stored all the data of the user mailbox like emails, attachments, contacts, notes, journals, etc. You can import the CSV file into various email clients and access the data. It helps to import the contacts into the account. If you want to know the best way to convert OLM to CSV then you can read the informative blog that provides every essential information Convert OLM to CSV.