OpenAnyFile
What is a .ost file?
Outlook Offline Storage Table is an email and contacts format developed by Microsoft, first released in 1997. It is usually created by Outlook with Exchange, Microsoft 365, IMAP.
Same NDB structure as PST but bound to one mail profile — copying an OST to another PC does not make it openable in Outlook.
File details
| Full name | Outlook Offline Storage Table |
|---|---|
| Category | Email & contacts |
| MIME type | application/vnd.ms-outlook |
| Signature (magic bytes) | 21 42 44 4E (!BDN) |
| Developer | Microsoft |
| Introduced | 1997 |
| Opens with | Universal inspector |
| Uploaded? | No — read locally in your browser |
How to open a .ost file
OpenAnyFile routes .ost files to its universal inspector, which shows a hex view, the detected signature, checksums and any readable text.
- Drop your .ost file on the box above, click it to choose the file, or paste it with Ctrl+V.
- The real format is detected from the file's first bytes — not just its name.
- The file opens in the universal inspector.
- Use fullscreen for a closer look, check its checksums, or download it back unchanged.
What works — and what does not
Works
Viewing and inspecting: a hex view, the detected signature, checksums and any readable text. Size, type, signature and checksums are always shown.
Does not work
A rendered view. This format has no open decoder, so you get a byte-level inspection and readable text instead.
Is it safe to open a .ost file online?
Here, yes — because nothing is uploaded. Most "online viewers" send your file to their server. OpenAnyFile reads it with your browser's File API and decodes it on your own device, so there is no copy anywhere else. You can check: open your browser's network tab while opening a file — no upload appears.
Questions about .ost files
Can I open a .ost file without installing software?
Yes. OpenAnyFile opens .ost files in your browser using its universal inspector. You get a hex view, the detected signature, checksums and any readable text, with nothing installed and no account.
How do I know a file is really a .ost file?
Check its first bytes instead of its name. A genuine OST file begins with 21 42 44 4E (!BDN). OpenAnyFile shows the detected type, so a renamed file is easy to spot.
Is my .ost file uploaded anywhere?
No. The file is read locally by your browser and never sent to a server — it even works offline once the page has loaded.
Does it work on phones?
Yes — on Android and iPhone, in any modern browser. You can also install OpenAnyFile as an app.
Related formats
Other formats from the same family:
Format facts checked and page updated on September 25, 2026.