OLM
The archive format Outlook for Mac exports to — a proprietary container that is not the same thing as Windows' PST, and that few other programs can read without conversion.
OLM is what Outlook for Mac produces when you use its Export command: a single proprietary container holding messages, attachments, contacts and calendar items. Despite serving the same purpose as PST on Windows, it is a different format, and a PST tool will not read an OLM file — a distinction that catches out anyone migrating between the two platforms.
Because the format is proprietary and supported almost nowhere outside Outlook for Mac, an OLM archive is a poor long-term home for your mail. If the application stops running on a future macOS, or the licence lapses, the archive becomes hard to read. Converting to an open format while you still have a working Outlook is the cheap insurance.
The usual route is to convert the OLM to MBOX or EML, after which any standard reader — Mbox Viewer among them — can open, search and export the messages. Our guide on converting Outlook PST and OLM archives to MBOX walks through the practical options.
Related terms
Personal Storage Table — Microsoft Outlook's container file for an entire mailbox, including folders, messages, contacts, and calendar items. Also known as OST when used for offline sync with Exchange or Microsoft 365.
A plain-text file format that stores multiple email messages concatenated together, each beginning with a "From " separator line. It is the format Google Takeout produces when you export your Gmail archive.
Microsoft Outlook's proprietary binary format for a single email message, storing headers, body, and attachments in a Compound Document Structure. Unlike EML, MSG is not a plain-text standard.