How to Compact OST Files without Opening Outlook in 2025 – Key Tips

The Outlook Data Files (OST and PST) are vital as they store large amounts of valuable information. However, as the data keeps getting stored in them, a situation arises when the files become oversized. At such times, it is essential to compact OST files for better data security with the right solutions. To do so, this blog describes reliable ways and a PST Compress Tool for reducing size without Outlook. 

What is an OST File?

Before you compress OST file, learn about its whereabouts. OST (Offline Storage Table) file is a copy file of Outlook data stored on the local system. It contains all Outlook items, from emails to calendars and contacts, tasks, journals, notes, and more. The OST file keeps expanding as we send and receive emails in Outlook, resulting in different errors. Some of them are as follows:

• Slow application performance
• Data inaccessibility 
• Synchronization issues
• No responses from Outlook 
• Incoming and Outgoing email issues

Thus, it is very important to compact OST file size timely to avoid losses. In addition, there are some more benefits of this OST file compression.

Reasons to Compact Outlook OST File

The significant advantages of reducing OST file size in MS Outlook are mentioned here:

• Outlook gives easy access to small OST files and can store more data.
• Shrink OST files to reduce the chances of file corruption or data loss.
• Increase the storage space in the Outlook mailbox by compacting files.
• Avoid deletion of emails and store them for a longer period in the inbox.
• Compacting helps reduce the occurrence of errors due to oversized OST.

Reliable Techniques to Compact OST File Size

Users can perform compression of OST files with three different manual methods or a professional tool as per their choice. A detailed explanation with complete steps for the methods is presented in this section. Let’s start right away to shrink OST files. 

Manually Reduce OST File Size in Outlook

Follow these methods and steps to overcome the issue of oversized OST in Outlook.

Method 1: Using Compact Now Feature

1. Start the desktop Outlook app and click the File tab from the top.
2. Then, press Account Settings and again choose it to open the Account Settings dialog.
3. Here, click the Data Files tab to show all the present OST files in Outlook.
4. Select OST files to compact and click Settings from the above options.
5. Further, open the Advanced tab and opt for Outlook Data File Settings.
6. Browse your OST file, click Compact Now, and lastly, hit OK.

This will compact Outlook OST file. However, if the OST size issue remains then try other solutions.

Method 2: Using Mailbox Cleanup Option

1. In your Microsoft Outlook, press File –-> Tools –-> Mailbox Cleanup.
2. A dialog opens. Here, press View Mailbox Size to check OST file size.
3. After checking, choose Find items older than or Find items larger than.
4. Now, delete the unwanted emails and items as per the found result.

Alternatively, you can also delete already removed items and alternate versions of mailbox items to free up space.

Method 3: Compress OST File With the Archive 

1. In newer Outlook versions like 2019, move to File and thereafter, click Tools.
2. Select the Clean Up Old Items option to open the Archive dialog box.
3. Later, select the OST folders and specify a date in Archive items older than.
4. Subsequently, click Browse and select the path to save the archive file.
5. At last, click OK to archive OST files that are old or not frequently required.

Drawbacks of the Manual Method

• Adding large no. of OST files to the archive folder can affect already stored PST files. This may lead to oversized PST.
• Deleted emails cannot be recovered again and any mistake can lead to corruption.
• The methods are not very effective and can also take a lot of time if the file is too large.

Dedicated Tool to Compact OST File or PST File Without Opening Outlook

The manual methods are there for OST compression and will be applied for PST files too. However, they are risky and don’t guarantee that files will surely be compressed. Therefore, a better solution to compact PST files and OST files is the Cigati PST Compress Tool. This excellent utility can simultaneously handle multiple Outlook data files and perform their compression within seconds. Additionally, it is easy to use and has no size limitations for all tech and non-tech users. Moreover, users can utilize other distinctive features of the tool such as:

• Compress and save attachments and other OST file data in a desired folder.
• Allows to separate attachments from OST and PST files during compression.
• Preview the Outlook mailbox items and reduce file size securely with ease.

Steps to Compact PST File and Shrink OST

1. Launch the Outlook Compress Tool and choose the file type as OST.
2. Select the mode of file and click Next to add files to the tool.
3. Choose OST files from the system, click Open to upload, and then, hit Next.
4. Expand the OST folders to preview and select for compression. Tap Next.
5. Afterward, choose a compression option for attachments and other filters.
6. Click Path to Save and browse a destination to save compressed OST.
7. At last, hit Start Compression to initiate the process of reducing the OST file.

Conclusion

This blog elaborated on different reasons and methods to compact OST files and PST files. Maintaining the size of Outlook data files is a task that must be handled very carefully. You can use the manual method to reduce oversized OST files but it has some limitations. Therefore, opt for the best professional tool for secure compression without hassle.

Also Read : Convert OST File to DOC.