5 Effective Ways to Solve Oversized OST File Issues

The size of an Outlook data file can significantly impact Outlook performance. According to Microsoft, an Outlook data file (.OST) up to 5 GB provides a good user experience on most systems. However, if the file size grows to 5-10 GB, Outlook requires a system with decent hardware configuration, such as faster storage media (SSD) and more memory (RAM). 

When the ost has reached the maximum size, users start experiencing Outlook issues such as Outlook freezes or not responding while using the email client. Although these pauses or freezes occur for a short duration, they are annoying and affect your work. But when the Outlook data file exceeds the 20 or 25 GB size, which is the default maximum and recommended limit, the Outlook performance significantly decreases. 

A large or oversized Outlook OST data file is known to cause Outlook performance issues and lead to problems, such as frequent Outlook freezes, Outlook crashes, delays in sending or receiving emails, synchronization errors, etc.

In some cases, the OST file becomes inconsistent and inaccessible, resulting in Outlook data file errors, such as xxx.ost is not an Outlook data file (.ost).


This prevents users from accessing their emails and may lead to a data loss situation. Thus, it is vital to check the OST file size and prevent it from oversizing to avoid the aforementioned issues.

Ways to Solve Oversized OST File Issues

There are different ways that you may follow to reduce oversized OST file size and resolve the Outlook issues. But before following these, check the OST and mailbox size.

In Outlook, right-click on your profile name and choose Open File Location….

Right-click on the [email protected] file and choose Properties.

Check the OST file size. If it is large-sized or oversized, follow the ways explained below to reduce the OST file size.

1. Clean up Mailbox

There are different approaches to clean up the Outlook mailbox. You can use the Mailbox Cleanup option to keep your mailbox tidy and reduce the mailbox (.ost) size by removing unnecessary emails from your Inbox. You may also reduce the OST file size by manually deleting the emails from Deleted Items and Spam or Junk folder. 

The steps are as follows:

  • In Outlook 2016 and later, go to File > Tools > Mailbox Cleanup….
Mailbox Cleanup
  • In Outlook 2013, go to File > Cleanup Tools > Mailbox Cleanup….
Mailbox cleanup tool
  • Empty the Deleted Items folder and delete the conflicts (alternate versions of items in your mailbox). 
  • Also, find items older than certain days or larger than a certain size and move them to a local PST file.
  • Select all the emails found using CTRL+A.
  • Right-click and choose Move > Copy to Folder… 
  • Select the folder in your local PST file and click OK.

The mail items will be moved from the oversized OST file to the local PST folder.

2. Archive Mail Items

You can set up Outlook to auto-archive emails or archive emails manually based on your needs. Auto-archive enables Outlook to create an archive at a regular interval. It is the most effective and easiest way to deal with large or oversized OST files and keep the file size under recommended limits. 

To archive old emails and reduce the OST file size manually, follow these steps:

  • In Outlook 2016 and later, go to File > Info > Tools > Clean up old items.
  • In Outlook 2013, go to File > Info > Cleanup Tools > Archive.
  • Click Archive this folder and all subfolders.
  • Select the folder to archive, such as Inbox, and select the date in Archive items older than.
Archive dialog box
  • Click Browse to choose a save location and then click OK

Outlook will archive all mail items older than the specific/selected date and save them in a separate PST file that you can access locally. 

3. Disable Download Shared Folders Option

When Outlook is configured to download shared folders, the mail items downloaded are stored in the default OST file. Thus, if the shared folders contain multiple or large emails/attachments, it can significantly increase the OST file size. 

To avoid this, disable downloading Shared folders in Outlook by following these steps:

  • Open Outlook and go to File > Account Settings > Account Settings.
  • Select the profile and click Change.
  • Click More Settings and go to the Advanced tab.
  • Uncheck the Download shared folders option and click OK.
  • Restart Outlook for the changes to take effect.

4. Compress Oversized OST

Outlook provides an option to compact OST files, which essentially removes the whitespaces left behind by the deleted mail items, to reduce the OST file size. This can help keep the OST file size under check if you often delete unnecessary or spam/junk emails from your mailbox.

The steps are as follows:

  • Open Outlook and go to File > Account Settings > Account Settings.
  • Select the profile and click Change.
  • Click More Settings and go to the Advanced tab.
  • Click Outlook Data File Settings… and then click Compact Now.
  • The Outlook will compact the OST. Then, check the OST file size.

If you don’t see much change in OST file size, you can use an advanced OST converter software to compress the OST file. Stellar Converter for OST is one such tool that can help you significantly reduce the Outlook OST file size. The software compacts the OST by removing or saving the attachments in a separate folder and interlinks them to allow users to access them from within the email. 

5. Split Oversized OST

If you want to split a large or oversized OST file into smaller files manually, you need to export mail items from the OST file to PST multiple times and for each mail folder. This isn’t an efficient way to split a large OST file. 

Instead, you should use an OST to PST converter tool, such as Stellar Converter for OST, to convert the OST file and split it into multiple smaller PSTs based on the size (1-5 GB each) or a date range. 

Besides, the software automatically splits a mail folder if it detects the number of mail items exceeding the 100,000 items/folder threshold limit to ensure consistent Outlook performance while retrieving mail items from the Outlook data files.  

Quick Tips to Keep OST File Under Recommended Size Limits

Follow these quick tips to prevent OST files from overgrowing or oversizing, 

  • Archive older or larger mail items once in a while.
  • Empty Deleted Items and Junk/Spam folders regularly.
  • Store attachments in a separate folder outside your OST mailbox on your system.
  • Compact the OST file to remove the whitespaces and reduce the file size.
  • Use cloud storage, such as OneDrive or Google Drive to share large or multiple attachments. You may also use your organization’s local cloud storage or network storage for sharing attachments. 


In this article, you learned five different ways to solve oversized OST file issues in Outlook. These will work for all Outlook versions, such as Outlook 2010, 2013, 2016, 2019, and 2021. However, if these tips fail to fix or resolve the issue, you can always use an advanced OST to PST converter software, such as Stellar Converter for OST, to resolve the oversizing issue. The software helps you retrieve mail items from an inaccessible or damaged OST file to PST format. It compresses and splits the OST file to keep its size under recommended limits and avoid damage or data loss.

Christopher Stern

Christopher Stern is a Washington-based reporter. Chris spent many years covering tech policy as a business reporter for renowned publications. He has extensive experience covering Congress, the Federal Communications Commission, and the Federal Trade Commissions. He is a graduate of Middlebury College. Email:[email protected]

Related Articles

Check Also
Back to top button