OST file is also known as Offline Storage Table. OST file extension is used by Microsoft Outlook application. This file is stored on local computer of user, which allows user to access his/her mails and other Outlook data without having an internet connection. You can read, write an email or you can also view and create an appointment without having an internet connection. However, your emails won’t be sent and you won’t receive any new emails until your internet connection is restored. After the restoration of internet connection, all changes made during offline mode will synchronized.
OST file is similar to PST file type, but the main diffrence is that OST is only used for storing offline data

OST file is created when “Use Offline Folders” option is selected is MS Outlook application.

• Default location of OST file is located in the directory mentioned below.
  \Users\user\AppData\Local\Microsoft\Outlook
• You can also view OST file by navigating File> Account Settings> Account Setting then Select Data File tab and then finally click on Open File Location.

There are many possible ways to Open or view OST file, some of them are as follows.

Method 1.

Convert OST file to PST file format using Advik OST to PST Converter and then import PST file in Outlook

Method 2.

You can also view and import OST file to Gmail or Yahoo mail using these tools
Advik OST to Gmail Import
Advik OST to Yahoo Import

Method 3.

Using Free OST Viewer

1. SysTools OST Viewer Freeware

2. BitRecover OST Viewer