Getting an .olm into Outlook for Windows (and a .pst out)

The file you get

From this site, a .zip of .eml files, one per message. From Outlook, after the import, a .pst containing the same mail, produced by Outlook's own Import/Export wizard.

Steps

1. Get the .olm out of Outlook for Mac first: Tools tab → Export, tick what you want, Continue, save the file. If Export is greyed out you are in the new Outlook; switch the New Outlook toggle off, top right, and it comes back.
2. Drop the .olm on this page. It is read by JavaScript in your browser tab; the file never leaves your machine.
3. Export to EML and unzip the result. One .eml per message, in folders matching the Outlook tree.
4. In classic Outlook for Windows, create a new folder (right-click your mailbox → New Folder) and name it for the archive.
5. Drag the .eml files into that new folder, in batches of about 20 to 30. Dropping them onto the Inbox can make Outlook treat them as attachments instead of messages, so always drop into a folder you made. Widely reported to work; Microsoft does not document .eml import, so check that the first batch arrived as real messages before doing the rest.
6. Export the folder: File → Open & Export → Import/Export → Export to a file → Next → Outlook Data File (.pst) → Next. Pick the folder, tick Include subfolders, click Browse, name the file, then Finish. Set a password or leave it blank.
7. The .pst is now on disk, made by Outlook, openable by any Outlook.

What carries over

• Every message with its headers, body and attachments, through EML and into Outlook
• Folder structure, if you recreate the folders in Outlook and drop each folder's files into its own
• Original dates on the messages
• Everything Outlook itself then writes into the .pst on export

Good to know

• The reliable alternative to drag-and-drop, and the better route for thousands of messages: convert to MBOX, import that into Thunderbird (Local Folders), add the destination mailbox to Thunderbird as IMAP, and drag the folders onto it. The mail goes up to the server; open the same account in Outlook for Windows and export to .pst from there. Slower, but it does not depend on undocumented drag behaviour.
• New Outlook for Windows cannot import a .pst and does not handle calendar or contact items stored in one. Use classic Outlook for both the import and the export.
• Contacts and calendar do not travel as EML. Export them separately as .vcf and .ics and import those into Outlook.
• Outlook writes times in UTC but leaves the time zone off the timestamp. Every date here is read as UTC, so nothing shifts by your offset, a common fault in other converters.
• Microsoft has never published the .olm format. Mail, attachments, contacts and calendar entries are read from the XML Outlook writes; unusual items (notes, tasks, journal entries) are skipped in this version.