How to Compact OST File in MS Outlook 365?

In today’s digital era, efficient data management is crucial, especially when it comes to email storage. Since Microsoft Outlook is a widely used application for email management, users often deal with large OST (Offline Storage Table) file complications, impacting system performance.

When the size of an MS Outlook data file increases above a certain limit it not only affects the performance but might also lead to data file corruption. Also, OST file size is the most common cause of OST file errors as well as frequent Outlook crashes. To prevent it; the blog will guide you on how to shrink, reduce, and minimize your OST file size through the compacting process in Outlook 365.

Why Compact Your OST File?

Compressing an OST file is essential for maintaining Outlook performance and reducing the risk of OST file errors. Over time, as you send, receive, and delete emails, your OST file accumulates unnecessary data, leading to increased file size. That slows down the speed and Outlook will stuck on loading the profiles every time you try to open it. 

Shrinking helps to reclaim storage space, reduces the risk of file corruption, and enhances overall Outlook speed. Thus, whether it is an OST file or a PST file if the size grows, it is best to keep an eye on it and compact the PST file size after a certain limit.

How to Compact OST File in Outlook 365?

MS Outlook provides an inbuilt app to directly compact the OST file size. Although compacting the size of an OST file manually is often very difficult, when you follow these step-by-step instructions, you can do it smoothly:

Step 1: Open Outlook 365

  • Launch your Outlook 365 application on your computer.

Step 2: Navigate to the Account Settings

  • Click on the “File” tab in the top-left corner.
  • Select “Account Settings” and then choose “Account Settings” again from the drop-down menu.

Step 3: Select Data Files

  • In the “Email” tab, highlight your email account and click “Settings.”
  • Go to the “Data Files” tab.

Step 4: Initiate Compact Process

  • Choose the data file (OST) you want to compact and click “Settings.”
  • In the “General” tab, click “Compact Now.”

Step 5: Monitor Compact Progress

  • A progress bar will appear, indicating the status of the compacting process.

Step 6: Completion and Verification

  • Once the process is complete, click “OK” to finish.
  • Verify the reduction in file size by checking the file properties.

It is the only easy way to compact OST files. You cannot reduce the size of an OST file by any other means. Alternatively, you can try to keep the inbox clean and junk folder empty, but that too does not ensure the size reduction because the Outlook data files are designed in such a way that every data that you delete by command will leave white space in the file and never gets overwritten or removed to free up the space. That is why it is suggested to shrink the OST file size by using a compact OST file process every now and then as described above to protect the Outlook from frequent crashes.


Regularly compacting your OST file is a proactive measure to maintain a healthy and efficient Outlook environment. By following these simple steps, you can shrink, reduce, and minimize your OST file size, ensuring optimal performance and streamlined data management in Outlook 365. Remember to make compacting a routine part of your email maintenance to enjoy a clutter-free and efficient Outlook experience.

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