100% Private & Secure

Free OST Viewer
Inspect Offline Outlook Files

Drop an Outlook Offline Storage (.ost) file to inspect header, extract readable content, and browse email-like data. Same format as PST — no upload.

Header analysisFormat detectionHex dump viewNo Outlook required

Drop your .ost file here

or click to browse

About

What Is an OST File?

OST (Offline Storage Table) is Microsoft Outlook's format for caching an Exchange or IMAP mailbox locally. When you use Outlook in cached mode, it creates an .ost file that mirrors your mailbox. The file uses the same binary structure as PST: magic bytes "!BDN", format version (ANSI 14 or Unicode 23) at byte 10, and content type at bytes 8–9.

This free OST Viewer reads the file on your device and parses the header to show filename, size, format version, and type. It also shows a hex dump of the first 512 bytes. Full folder and message parsing requires desktop software like Outlook.

Use the viewer to quickly verify an OST file, check ANSI vs Unicode, or inspect structure before migration or recovery — all without uploading your data.

Features

Features of the OST Viewer

Inspect Outlook OST files with no install

Header parsing

Read OST/PST magic bytes (!BDN), format version (ANSI/Unicode), and content type from the file header.

OST vs PST

OST uses the same file format as PST. The viewer identifies the file type from the extension.

Hex dump

View the first 512 bytes in a monospace grid: offset, hex, and ASCII for structure inspection.

No upload

OST files are processed entirely on your device. Nothing is sent to any server.

Private & secure

Your OST file never leaves your device. Ideal for quick structure checks without desktop tools.

Works everywhere

Use on Windows, Mac, or Linux. No Outlook or OST desktop software required.

How to View OST File Structure Online

Four simple steps — no install, no sign-up

01
01

Open the OST Viewer

Go to the OST Viewer page and you'll see the upload area.

02
02

Drop your .ost file

Drag and drop an Outlook .ost file, or click to browse and select one.

03
03

View header & metadata

Filename, size, format version (ANSI/Unicode), and type are shown in a clean info card.

04
04

Inspect hex dump

Review the first 512 bytes in offset | hex | ASCII layout for structure analysis.

Use Cases for the OST Viewer

When and why to inspect OST files online

Offline mailbox check

Verify an OST file (cached Exchange mailbox) without opening Outlook.

OST to PST migration

Inspect OST structure before converting or migrating to PST or another system.

Support & recovery

Confirm file type and format for support or recovery without uploading full content.

No Outlook installed

Check OST files on a machine that doesn't have Microsoft Outlook or Exchange cache.

Developer inspection

Inspect OST header and raw bytes during development or debugging.

Backup verification

Confirm backup or copied OST files are valid before recovery or migration.

Compare Plans

Free vs Paid — OST Viewer

Get started free, upgrade when you need more power.

Feature
Free
Paid
Daily usage
5 uses/day
Unlimited
File size limit
10 MB
Up to 500 MB
All core features
No software installation
Works on any device
Files stay on your device
Batch processing
Priority support
Upgrade — starting at $6.99/mo
Upgrade to Full Version

Unlock the Full Power of OST Viewer

Remove daily limits, process larger files up to 500 MB, enable batch processing, and get priority support.

Buy Full Version — $6.99/moor $300 enterprise

File Viewers includes:

  • 21 file viewers included
  • Unlimited daily uses
  • 100 MB file size limit
  • All file formats supported
  • Email analysis tools

Also available in the All Tools Bundle

FAQ

Frequently Asked Questions

Common questions about the OST Viewer

Q1. What is an OST file?
OST (Offline Storage Table) is Outlook's format for caching an Exchange (or IMAP) mailbox locally. It uses the same binary structure as PST (magic !BDN, ANSI/Unicode). OST files are created when you use Outlook in cached mode. Full parsing requires desktop software; this viewer shows header and hex dump only.
Q2. Do you upload my OST file?
No. The OST Viewer processes everything on your device. Your file never leaves your computer. Nothing is sent to our servers.
Q3. Why can't I see my emails?
Full OST parsing (folders, messages, attachments) is not possible with this tool. We provide file structure analysis and header information. For full content, use Outlook or a dedicated OST desktop tool.
Q4. What's the difference between OST and PST?
OST is for offline/cached mailboxes (often Exchange); PST is for personal storage and archiving. The file format (header, B-trees) is the same; the viewer distinguishes them by file extension.
Q5. Is the OST Viewer free?
Yes. The tool is free to use with no sign-up and no limits.
Q6. What if the header shows invalid?
The file may not be a standard OST/PST, could be corrupted, or truncated. Check the hex dump to verify the first bytes are 21 42 44 4E (!BDN).
Q7. Does it work on Mac?
Yes. The viewer works on Windows, Mac, and Linux. No Outlook required.
Q8. Can I view PST files here?
This tool accepts .ost files only. For PST files use our PST Viewer; the header structure is the same.
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