What is an OST file?
An OST file (Offline Storage Table) is a local copy of your Outlook mailbox. It lets you access emails, calendar items and contacts even when you’re offline. The original data remains on the server. The OST file simply enables offline access and later synchronisation. This guide explains how OST files work, where they’re stored and how they differ from PST files.
What is an OST file?
An OST file (Offline Storage Table) is automatically created in Outlook when you use an Exchange, Microsoft 365 or IMAP account. It stores a synchronised local copy of your mailbox so you can read and manage emails, appointments and contacts without an internet connection. Any changes you make offline are synced with the mail server the next time you connect.
In current versions of Microsoft Outlook, your mailbox data is cached in OST files. Outlook downloads your data from the server and keeps it temporarily in a local OST file for offline use. The original data is stored centrally and securely on the mail server.
Recent versions of Outlook offer faster and even more efficient offline caching, especially when working with large OST files. In Microsoft 365 environments that include cloud archives, OST files tend to be smaller because less data needs to be stored locally.
Note: This article covers Outlook (Classic). In the new Outlook for Windows, offline storage and PST support may differ.
Microsoft Outlook helps make everyday work easier for millions of users. It organises and stores emails, calendar items, journals and tasks so they’re available even when you’re offline. Other helpful Outlook features include the ability to recall emails sent by mistake or before they’re complete and to recover messages that have been deleted by accident.
What is the purpose of OST files?
Outlook uses OST files for Exchange, Microsoft 365, IMAP and Outlook.com accounts so you can read, edit and organise your emails even when you’re offline. PST files can also be modified without an internet connection, for example when you move or delete messages, but only OST files sync those changes with the server once you’re back online.
- The world’s leading email and calendar solution
- Secure hosting from a single source
- Anytime mobile access via the Outlook app
What are the advantages and disadvantages of OST files?
| Advantages | Disadvantages |
|---|---|
| Access to emails, calendars and contacts offline | Linked to a specific account/profile; not portable |
| Automatic synchronisation with the server | Repair options are limited when the file is damaged |
| Minimal data loss since the server copy remains the master | Large OST files can slow down Outlook |
| Suitable for multi-device use (the server keeps all data consistent) | Local folders can be lost if the file becomes corrupted |
| Central storage on Exchange/Microsoft 365 boosts security | Risk of data exposure if the device is lost and encryption has not been enabled |
A key benefit of using OST files is central server storage. Because your account data is stored on the mail server, you can access it from multiple devices and locations. If your computer fails, only the cached OST file is lost – the data on the mail server remains safe. If, on the other hand, the OST file is missing or damaged, simply reconfigure your Outlook account and allow it to resynchronise with the server to generate a new copy.
Since OST files are stored locally, they can be affected by device loss, malware or unauthorised access. To stay protected, use local encryption, make regular backups and consider backing up important folders or drafts externally.
What’s the difference between OST and PST files?
A PST file (short for ‘Personal Storage Table’) is another way to store Outlook data locally. Outlook downloads your data from the mail server and saves it in a PST file on your device. An OST file, on the other hand, is a cached offline version of your mailbox, with the original data remaining on the server.
The type of file Outlook uses depends on your account configuration. POP accounts download emails from the server using the POP3 protocol and store them as PST files. IMAP accounts use OST files as local caches.
One major difference is that changes made to a PST file change the original data. If a PST file is deleted or corrupted, the original content is lost and can only be restored with backup or recovery tools. OST files, on the other hand, sync their offline edits with the server the next time you’re online.
In older Outlook versions, especially 2013 and 2016, certain IMAP or Outlook.com folders were labelled ‘This computer only’. These folders were stored exclusively on your device and lost if the account was changed or removed. This feature is largely obsolete today and only relevant in older versions of Outlook.
Comparing OST and PST files
| Feature | OST file | PST file |
|---|---|---|
| Storage location | Local copy of a server mailbox (Exchange, Microsoft 365, Outlook.com, IMAP) | Primary local file, mainly used with POP accounts |
| Synchronisation | Offline changes sync with the server when online again | Changes apply directly, no server synchronisation |
| Use case | Offline work; data is stored on the server | Archiving or backing up data locally to free up mailbox space |
| Account dependency | Requires an active, linked account/profile | Can be opened independently of the server |
| Risk | Low because the server holds the ‘master’ copy | High, loss or damage means data loss |
| Typical scenarios | Working with Exchange/Microsoft 365 or IMAP in cached mode | POP accounts, long-term archiving or freeing mailbox storage |
A key advantage of PST files is that they free up mailbox storage. You can move data into a linked PST file, archive Outlook emails and reduce your server mailbox size.
- Professional, automatic archiving
- Securely stored in European data centres
- Protect yourself against data loss
Where are OST files located?
You can find the folder that contains your Outlook data files (both PST and OST) as follows:
- Step 1: Open the File tab in Outlook.
- Step 2: Click Account Settings and then Account Settings again.
- Step 3: Under the Data Files tab, select an entry and click Open File Location….
By default, OST files are located in the folder <Drive>\Users<Username>\AppData\Local\Microsoft\Outlook. OST files can only be opened with an active Outlook account linked to the same profile. In newer Outlook versions, the file location can’t be changed directly through the interface but can be modified via the ForceOSTPath registry key.
Outlook isn’t the only tool that supports online and offline mail synchronisation. Free alternatives to Outlook include Thunderbird, SeaMonkey or Mailbird.