What is the OLM file format?
.olm is a proprietary archive format used by Microsoft Outlook for Mac to export and back up mailbox data. .olm files store email messages, calendar data, contact data, and tasks - it is the macOS counterpart of the .pst format for Windows, introduced with Outlook for Mac 2011. Internally, an .olm is a ZIP archive containing an Accounts/ folder tree of XML message records and embedded attachments; renaming the file to .zip lets any archive tool inspect its raw contents.
Relation between .olm and .pst files
Although .olm and .pst files perform the same function and store the same type of data, each format is only supported on the platform it was designed for. Unlike .pst, which Windows Outlook can keep open as a live data store, an .olm is strictly an export snapshot. An .olm file can be converted to a .pst file and vice versa using tools such as Aid4Mail, but this should only be used as an interim solution during platform migration - for ongoing cross-platform access, keeping mail on an IMAP or Exchange server is a more reliable approach.
Security & safety
RISK: MEDIUMAn OLM is data, not an executable, so the file itself won't run - but it is a whole mailbox and can contain many emails with the usual threats (malicious attachments, phishing links) that only fire when a message is opened in a live client, plus highly sensitive personal/business data, so treat exports as confidential. The OLM format has no built-in encryption. The biggest practical risk is the software around it: searches for 'free OLM viewer/converter' surface adware-bundled installers and fake 'recovery' tools - download converters only from reputable vendors' official sites and scan the installer.
Format details
in a nutshellPrograms that open OLM files
Windows2 apps
Reputable OLM-to-PST converter (e.g. SysTools / Stellar Converter for OLM)
Paid Windows Outlook can't open an .olm - convert it to PST first, then open the PST in Outlook (File > Open & Export). Use a reputable vendor; avoid adware 'free viewers'. 
macOS2 apps
Aid4Mail
Paid Reads an .olm without Outlook to view and convert messages to MBOX/EML/PST for other clients. 
Microsoft Outlook for Mac
Paid Native route: File > Import > 'Outlook for Mac archive file (.olm)', pick the file; the folders appear under On My Computer. 
Linux1 app
Convert to MBOX, then any client
Open-source No native Linux .olm reader; convert the .olm to MBOX (Aid4Mail or a converter) and import into Thunderbird/Evolution via ImportExportTools. Technical details
deep spec| Container format | ZIP archive (magic bytes 50 4B 03 04 - "PK" - at offset 0) |
| Internal encoding | UTF-8 XML message records with inline or attached binary parts per ZIP entry |
| Compression | DEFLATE per ZIP entry; the container itself is not additionally compressed |
| Encryption | None - the .olm container has no built-in password protection or encryption layer |
| MIME type | application/octet-stream |
| Platform origin | Created exclusively by Microsoft Outlook for Mac on macOS; not writable by Windows Outlook |
| Windows Outlook compatibility | Cannot open .olm natively; requires a dedicated OLM-to-PST converter tool |
| Internal directory layout | Accounts/<account>/ subtree mirrors the mailbox folder hierarchy; messages stored per folder as XML files |
| Data types stored | Email messages (headers, body, attachments), calendar items, contacts, tasks |
| File identity method | Recognized by its internal Outlook-for-Mac XML schema, not a unique magic number - the ZIP signature (PK) is shared with all ZIP files |
| Typical export size | 10 MB to several GB, depending on total mailbox volume |
| Archive entry limit | 65,535 entries under standard ZIP; ZIP64 extension lifts this for large mailboxes |
| Per-entry integrity | CRC-32 checksum per ZIP entry; no message-level cryptographic hash or signature |
| Distinction from .pst | .olm is an export snapshot only - it is not a live, mounted data store the way Windows Outlook uses .pst |
| Predecessor format | Replaced the Entourage .rge database as the Mac Outlook archive and export container |
| Cross-platform migration | Can be converted to .pst (Windows) or .mbox (IMAP clients) via third-party tools such as Aid4Mail or SysTools OLM Converter |
| Released | 2011 (Microsoft Outlook for Mac 2011, replacing Entourage) |
| Latest version | N/A (used by current Outlook for Mac as the archive/export container) |
| Specification | support.microsoft.com |
OLM conversions
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