File Extension Library

.OST File Extension

  • Developer by: Microsoft
  • Category: Misc Files
  • Format: Binary

What is an .OST file?

.OST file extension is created by Microsoft. .OST has been classified as Misc Files. The format of .OST file is Binary.

.OST is Outlook Offline Data File

An OST file is a data file created by Microsoft Outlook (2005 or later), an email client and personal information manager. It is used to store a copy of mailbox information from an Office 365, Exchange, or account. OST files may contain email messages, contacts, tasks, calendar data, and other account information.

OST files are stored on the local computer of a user, which allows the user to access his emails and other mailbox information when he does not have Internet access. The user can look up and create appointments, search or add contacts, and read and write emails when offline. However, any changes made offline will not be synchronized with the user account on the server until an Internet connection is re-established.

An OST file is created when the "Use Offline Folders" option is selected in Outlook. The file will be located in the following directory:


You can also navigate to the location of the OST file by selecting File → Account Settings, clicking Account Settings..., selecting the Data Files tab, clicking an entry, and then selecting Open Folder Location.

NOTE: The OST file is similar to a Personal Folder (.PST) File, but is only used for storing offline data.

List of all softwares that can open the Outlook Offline Data File
Microsoft Outlook 2016
Remo Convert OST to PST Outlook Recovery Toolbox
4Team Corporation OST PST Viewer
Aryson OST Recovery

How to Open .OST file on different platforms

Open .OST File on Windows

  • Right-click a .OST
  • Left-click "Open With" > "Choose another app"
  • Select the program that opens .OST file and Select check box "Always use this app to open .OST files" Press "Ok"

Open .OST File on Mac

  • Right-click (or Ctrl + Left-click) the .OST file
  • Click "Open With" > "Other.."
  • Then choose another program from the app list and press "Open"
  • Select check box "Turn on" for "Always open in app"

Open .OST File on Linux

Right click on the .OST file, choose "Open with" and select a program from the list above

Open .OST File on iPhone

Select a file. If you don't see an application to open the .OST file, tap "Share" and choose the right app.
Method 2
Save the file. Then open the app «Files». Touch and hold the file, then release your finger. You will see a black menu bar. After that tap "Share" and select an application.

Open .OST File on Android

Tap the Settings icon on an Android smartphone, then select «Applications». Click the Settings icon in the upper right corner of the screen, to change the default apps.
If you want to open, edit, save or even convert .OST files on Android, then your best bet is to download one of the dedicated apps available on Google Play.

How to fix problems with .OST files

  1. You need to update the application that you normally use to open .OST files. Only the latest version of the software supports the current .OST file format
  2. You need to check the .OST file for viruses. To do this, you need to scan it with a popular antivirus (Norton, Nod32, Kaspersky, Dr.Web, etc.)