If you use Microsoft Outlook for work, business, or personal emails, you’ve probably come across an .OST file at some point. Many users notice this file taking up large space on their computer but don’t fully understand what it does, why it exists, or how to open it safely.

In today’s modern workplace, where email access needs to work both online and offline, Outlook relies on a special file format called the Offline Storage Table (.OST). This file allows you to continue reading, writing, and managing emails even when you don’t have an internet connection.

Whether you’re troubleshooting Outlook issues, recovering emails, migrating data, or simply trying to understand how Outlook stores your mailbox, learning about OST files is extremely important.

What Is an OST File?

An OST file (Offline Storage Table) is a local copy of your mailbox data created by Microsoft Outlook when you use an Exchange, Microsoft 365, or IMAP account.

Think of it as a mirror copy of your mailbox stored on your computer.

OST File Stores:

Emails
Attachments
Contacts
Calendar events
Tasks
Notes
Folder structure

Outlook syncs this file with the mail server whenever internet access is available.

Why OST Files Exist

OST files are designed for offline access.

Without OST, you could only access your emails while connected to the mail server. With OST, Outlook works even without internet.

This is called Cached Exchange Mode.

OST vs PST File (Important Difference)

Many users confuse OST with PST files. Here’s a simple comparison:

Feature	OST File	PST File
Full Name	Offline Storage Table	Personal Storage Table
Internet Required	No (works offline)	Not required
Syncs With Server	Yes	No
Used With	Exchange / Office 365 / IMAP	POP3 & Manual backups
Can Be Imported	No (directly)	Yes
Portable	No	Yes

Key takeaway:
OST is a server-linked cache, PST is a portable backup file.

How Does an OST File Work?

When you add an Exchange or Microsoft 365 account to Outlook:

Outlook connects to the mail server.
It downloads your mailbox locally.
It creates an OST file on your computer.
All changes sync automatically.

Example

Send email offline → stored in OST → synced later.
Delete email → removed from OST → server updated.

This ensures zero downtime email access.

Where Is the OST File Located?

The OST file is stored in the Windows user profile folder.

Default OST File Location:

C:\Users\YourUsername\AppData\Local\Microsoft\Outlook\

How To Find It Quickly

Press Windows + R
Paste:

%localappdata%\Microsoft\Outlook

Press Enter.

You’ll see your OST files here.

Why OST Files Become Very Large

OST files can grow very fast because they store full mailbox copies.

Common Reasons:

Large attachments
Long email history
Multiple mailboxes
Cached shared folders
Calendar & Teams data sync

Outlook OST files can grow 50GB+ easily in enterprise environments.

Can You Open an OST File Directly?

Short answer: No.

OST files are encrypted and linked to the original Outlook profile and mail server.

You cannot simply double-click and open them like documents.

But don’t worry — there are safe methods to access the data.

How To Open an OST File (All Working Methods)

Method 1 — Open OST via Outlook (Best Method)

If the Outlook profile still exists, this is the easiest way.

Steps:

Open Outlook
Go to File → Account Settings → Account Settings
Select Data Files
Choose the OST file
Click Open File Location

Outlook will automatically load and sync the data.

✔ Works when profile is active.

Method 2 — Convert OST to PST (Most Popular Method)

Since OST files cannot be imported directly, converting to PST is the best solution.

Why Convert OST to PST?

Open on another PC
Backup emails
Recover deleted profile
Access orphan OST file

Convert OST to PST Using Outlook (Free Method)

Works only if Outlook profile is still connected.

Steps:

Open Outlook
Click File → Open & Export
Select Import/Export
Choose Export to a file
Select Outlook Data File (.pst)
Choose folders and export.

Your PST file will be created.

Other Tools to Open OST Files (Professional Solutions)

If Outlook profile is deleted or the OST file becomes orphaned, you’ll need third-party tools to open or convert it.

Below are reliable software options:

1. Stellar Converter for OST

Overview:
A professional-grade tool that allows you to open, preview, and convert OST files to PST, EML, MSG, HTML, and PDF formats.

Key Features:

Opens corrupted or orphan OST files
Preview emails before conversion
Supports large OST files (50GB+)
Maintains folder structure

Best For: IT professionals & enterprise recovery.

2. Kernel for OST to PST

Overview:
A popular OST recovery tool known for handling severely corrupted files.

Key Features:

Converts OST to PST
Recovers deleted emails
Supports Office 365 migration
Maintains data integrity

Best For: Corrupted or inaccessible OST recovery.

3. SysTools OST Recovery

Overview:
An advanced OST viewer and converter with powerful scanning technology.

Key Features:

Open and preview OST without Outlook
Convert to PST/EML/MSG
Handles encrypted OST files
Split large PST files

Best For: Users without Outlook access.

4. Aryson OST to PST Converter

Overview:
A lightweight tool suitable for small business users.

Key Features:

Supports ANSI & Unicode OST
Preview before export
Converts to multiple formats
Simple UI

Best For: Small-scale recovery needs.

5. Free OST Viewer Tools

Some free tools allow viewing OST content without conversion. However:

⚠ Most free versions:

Limit export size
Restrict saving options
Don’t support corrupted files

For critical data, paid tools are safer.

How To Convert OST to PST Without Outlook

If Outlook isn’t working:

Install any trusted OST Converter tool
Select the OST file
Scan mailbox data
Export as PST

This is the only method to recover orphan OST files.

Common OST File Problems and Fixes

1. OST File Corruption

Symptoms:

Outlook crashes
Emails not syncing
“Cannot open data file” error

Fix: Rebuild OST File

Steps:

Close Outlook
Go to OST location
Delete the OST file
Reopen Outlook

Outlook will rebuild a fresh OST.

2. OST File Too Large

Fix:

Disable shared folder caching
Archive old emails
Convert to PST backup

3. Outlook Not Syncing

Fix:

Check internet connection
Update Outlook
Recreate Outlook profile

Can You Delete an OST File?

Yes — and it’s safe.

OST files are temporary cached copies.

If deleted:

Outlook downloads data again from server.

⚠ Do NOT delete PST files though.

When Should You Convert OST to PST?

You should convert when:

Changing computer
Migrating emails
Recovering lost profile
Backing up mailbox
Closing company account

Benefits of OST Files

Offline email access
Faster Outlook performance
Auto sync with server
Reduced downtime
Full mailbox mirror

FAQs About OST Files

What program opens OST files?

Microsoft Outlook is the primary program. Third-party OST viewer or converter tools can also open them.

Is OST file important?

Yes, it stores your local mailbox copy.

Can OST be converted to PST?

Yes, using Outlook export or OST converter software.

Is OST a backup?

No. It’s a synced cache, not a backup.

Can I move OST to another PC?

Not directly. Convert it to PST first.

Final Thoughts

OST files play a crucial role in how Microsoft Outlook delivers seamless offline and online email access. While you cannot open them directly like regular files, they are extremely useful for caching mailbox data, improving performance, and preventing work interruptions.

If your Outlook profile still exists, exporting OST to PST is simple. If you’re dealing with an orphan OST file, professional conversion tools like Stellar, Kernel, or SysTools provide reliable recovery options.

Understanding OST files helps you troubleshoot Outlook, recover emails, and manage storage efficiently — an essential skill for both everyday users and IT professionals.