How to Backup OST Files in Outlook 365 [Windows 10/11]

What Is an OST File?

An OST (Offline Storage Table) file is a local, automatically created copy of your Outlook mailbox. When you connect Outlook to an Exchange, IMAP, Microsoft 365, or Outlook.com account, it downloads your emails, calendar, contacts, and other mailbox items into an OST file so you can work offline.

Once reconnected to the internet, Outlook syncs any changes in the OST file back to the mail server. This setup boosts performance, reduces server load, and ensures you can still read or draft emails even without a network connection.

OST vs. PST: What’s the Difference?

While an OST file looks similar to a PST file at first glance, they serve completely different purposes:

Feature	OST	PST
Purpose	Local cache of the server mailbox	User-created backup/export file
Created by	Outlook (automatic)	User (manual export)
Restorable?	No, tightly tied to the original account/profile	Yes, fully portable
Sync method	Two-way sync (between local and server)	No sync; standalone file

In fact, the OST file is not a true backup, just sync email from or to the server. It becomes unusable if:

• Your Outlook profile has been deleted.
• Your account has been removed.
• The server mailbox changes.
• You move to a different device.

On the other hand, PST files are portable, independent, and fully restorable. Thus, many users want to backup OST to PST for safe, long-term email backup and recovery while continuing with OST files for fast offline access.

Why You Should Back Up OST Files?

Even though Outlook allows you to recreate an OST file from the mail server, it’s still vulnerable and may be corrupted or inaccessible due to disk failure, profile corruption, sync errors, malware, accidental deletion, etc. You need an OST backup when:

How to Backup OST Files in Windows 10/11

Here are 4 methods to backup OS files in Windows 10 or 11. Before starting, it’s necessary to know Outlook OST file location:

Outlook Version	Windows Version	OST File Location
2007	Vista, 7, 8, 8.1, 10, 11	C:\Users\\AppData\Local\Microsoft\Outlook
2010	XP	C:\Users\\My Documents\Outlook Files
2010	Vista, 7, 8, 8.1, 10, 11	C:\Users\\Documents\Outlook Files
2013, 2016, 2019, 2021	Vista, 7, 8, 8.1, 10, 11	C:\Users\\AppData\Local\Microsoft\Outlook

Method 1: Backup OST Files in Outlook 365 using Software [Fast & Auto]

AOMEI Backupper Professional is a comprehensive backup software for Windows that seamlessly integrates the Outlook Backup feature to backup OST files in Outlook 365 on a schedule. Here are some highlights of it:

Step 1. Download and install the Outlook client on your computer. Open it and log in with your account. Then, open AOMEI Backupper. Click Outlook Backup under Backup.

Step 2. You'll be asked to select the emails and destination.

Click Select Source to choose your Outlook account and OST files. 

Then, click the PC icon to select a local drive, external drive, network share, or NAS to save them.

Step 3. Click Schedule to set a backup frequency, such as daily, weekly, monthly, or even triggers. Also, click Options or Backup Scheme to set up or change the desired items. At last, click Start Backup to backup OST files automatically.

💡Tips:
◾ For long-term email protection, it’s suggested to enable automatic backup cleanup to delete old backups automatically.
◾ To avoid unauthorized access, you can choose to enable encryption for backups.

Method 2: Manually Copy OST Files (for Smaller Files)

This is the simplest method- just locate your OST file on the PC and copy it to another location. It’s quick, but not a true email backup.

Step 1. Close Outlook completely. Press Win + R to open the Run dialogue. Type %localappdata%\Microsoft\Outlook and press Enter.

Step 2. Find the file with .ost extension, right-click it, and select Copy. Then, paste it to another location, such as an external drive, cloud folder, network drive, etc.

Method 3: Backup OST to PST using Outlook Export (Seamless Integration)

Exporting OST to PST file is the safest and most reliable backup method. PST files are portable, restorable, and compatible with any Outlook profile or device.

Step 1. Open Outlook. Click File > Options> Open & Export and select Import/Export on the right panel.

Step 2. Select Export to a file and click Next.

Step 3. Choose a file type - Outlook Data File (.pst). 

Step 4. Select the folder you want to export and click Next.

Step 5. Click Browse... select a location to save it. Click Finish.

Method 4: Backup OST Files with Archive Option (Reducing Mailbox Size)

You can also use the Archive option to backup OST files - keep your active mailbox smaller while improving Outlook’s speed and performance, especially for large mailboxes.

Step 1. Open Outlook. Click File > Options > Mailbox Cleanup and select Archive.

Step 2. Check the Archive this folder and all subfolders, and select the desired folder you want to archive.

Step 3. Click Browse to select a destination for the archive file, and then click OK.

Further Reading: OST File Is Not Backed Up

If you are using Microsoft Outlook and set it to keep local copies of your items, but the OST file is not backed up, they are excluded from the VSS snapshot by default. You can choose to delete OutlookOSTsubkeyin the Registry.

Step 1. Press Win + R, type regedit, and press Enter.

Step 2. Navigate to the following path.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\BackupRestore\FilesNotToSnapshot

Step 3. Look for the OutlookOST subkey. Then, right-click the entry and select Delete. Once done, restart your computer to apply the changes.

Final Thoughts

It’s necessary to backup OST files for long-term data protection. Damaged OST files, failed drives, or corrupted Outlook profiles can all instantly disrupt your access to data - especially when certain items haven't yet synced to the server.

Exporting OST to PST is the safest and most reliable method in Outlook for creating genuine, portable backups without extra software. However, manually exporting the OST file each time is time-consuming and labor-intensive. So, it's best to set up scheduled backups using powerful backup software - AOMEI Backupper.