OST File Format – What is .OST Extension and How to Open OST File

OST (Offline Storage Table) file in one of the popular file format created by Microsoft Outlook email client. This file saves the backup of Exchange Server, Office 365 and Outlook.com. The OST file allow users to work offline in Outlook even if you are not connected to Office 365 or Exchange Server. The Offline work that you have done will be synchronized back when Exchange Server, Outlook or Office 365 is online. The OST file stores complete data of a mailbox including emails, contacts, notes, tasks and calendars etc.

So in this article we will deep dive into this OST file Extension. We will learn each and every minor details about OST file format including the possible ways of accessing the OST file. Please share this article with your friends if you are enjoying it.

When Outlook Creates OST File Extension?

There is an option in Outlook settings named as Cache Exchanged Mode. When you enable this option then Outlook creates the offline copy of mailbox in the form of OST file. To enable this option follow the steps mentioned below:

Step 1: Open Outlook Email Client and then click on File button from.

Step 2: In File, you will see an option of Account Settings, Click on this option and open the Account Settings.

Step 3: From the Email Tab, Click on Change and then check the option Use Cache Exchange Mode.

That’s all this will create the OST file

Why OST File Format came into existence?

As we all knows that Exchange Server, Outlook.com and Office 365 only works when you are connected to internet. In the absence of internet you would not be even able to read the emails. To overcome this issue Microsoft decided to develop a new file extension which allows user to download all emails into one file named as OST file. As the data is already downloaded so you can easily read emails, add contacts, create appointments etc. even when you are offline. All this work is synchronized back to Exchange Server when you are connected to internet.

What is the Location of OST file format in Computer?

You can find the OST file at the location given below:


You can also directly open the folder having the OST file from Outlook itself. For doing this, Click on File >> Account Settings >> Select Data Files >> Select the OST file whose location you want to open and then click on Open Folder Location. This will open the location of OST file.

How to Open, View or Read OST File Extension?

OST file can only be opened with the Outlook in which it was created. If due to some reasons your OST file has lost the connectivity from Outlook then you would not be able to open it in any Outlook (Not Even in which the OST file in created). So if you have some orphaned OST file and you want to open or read that particular file, you will need to convert it into the PST file format. Till now there is no manual method that allows opening or previewing the orphaned or disconnected OST file. You will need to use some third party application to perform OST to PST conversion. One such wonderful tool is CareUtils OST to PST Converter.


In this article we have discussed about what is OST file Extension, Why do we need OST file and How to Open the OST file. If you need any help or facing any kind of difficulty regarding the article please feel free to comment. We will try our best to help you. Hope you enjoyed the article. Please share it will your friends if you like it.

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