How to Deal With Large OST File [2026 Guide]

We can manage small files more efficiently. This golden rule applies in the arena of Outlook data files as well. A small file means proper and efficient management of data. A large OST file is a serious issue. It affects the performance of Outlook. If not maintained properly, it would cause operational issues. In some cases, it prevents users from accessing sensitive email communication and attachments.

Before moving forward, let’s decode why OST files becomes large. We are working in a time where every executive and individual receives multiple email messages on a daily basis. Such messages also contain large attachments. Some messages contain multimedia like images or large excel sheets with crucial data. Such messages consume loads of space provided. In the absence of some space-optimization mechanism, the OST file will grow massively. It will give multiple errors. The large OST can lead to server sync issues. In some cases, it will lead to app crashes. In nutshell, the user must be very careful about space optimization of the data file (.ost). It not only assures smooth operations of the email client but also prevents data corruption.

Now the question arises “How to Deal with Large OST File” We, as a user, need to look out at ways for the proper space optimization. OST is a highly-secured data file. It is hard to manage and recover as well. The best strategy is to convert OST to PST. It is one of the effective ways. It has two main advantages. Firstly, such conversion reduces server dependency. Secondly, the user gets a secured as well as a portable backup of the mailbox content. Outlook data file (.pst) is easy to manage as well. Check out this post to know why OST files grow large. The guide will also shed light on methods to deal with a large OST file safely.

Consequences of a Large OST File:

As discussed in the previous section, a massive data file (.ost) means multiple issues. Some of them are discussed below:

• Operational Issues: An oversized data file (.ost) causes multiple operational issues. The user experiences slow performance, difficulty in accessing mailbox data, delayed search, etc.
• Sudden Freezing: The email client crashes frequently.
• Syns Issues: The email client experiences data inconsistencies. It happens when the synchronization with Exchange/Office 365 is not happening because an oversized .ost file.
• Error Messages: The use experiences error messages. One of such messages that pops up because of a large OST is “cannot open default folders.”

What Leads to a Large OST?

Some of the factors that can lead to this issue are discussed below:

• Huge Influx of Emails: When a user deals in high volume of email messages, causing this problem.
• Large Attachments: When the email messages accompany large attachments like JPEG, PDF, Excel sheets, etc., causing this problem.
• Caching Setting: When the user opts for full caching mode, it can lead to this issue.
• Incomplete or Failed Syncs: Improper sync issues can also result in this error.

Methods to Deal with Large OST File

In this part of the guide, we will disseminate information regarding ways to fix the issue of a large OST file. Some of them are discussed below:

Fix 1: Compact OST Manually: The user can utilize built-in ‘Compact Now’ Option to reduce the file size. Follow the below instructions:

• Browse File
• Go to Account Settings > Account Settings
• Then press Data Files
• After this choose the OST data file
• Hit on the Settings tab
• Then navigate to Advanced > Outlook Data File Settings…
• Click Compact Now. Finish Ok to conclude

Fix 2: Utilizing Mailbox Cleanup Option: The second manual method on the list is using the Mailbox Cleanup option. Follow the below steps:

• Browse File
• Navigate to Info
• Browse Cleanup Tools icon
• Then select Mailbox Cleanup… from the dropdown
• A dialogue box will pop up
• Press on ‘View Mailbox Size…’ button
• After this the user needs to set rules. He can go for any of the below options:

Find items older than: The user can discover and remove email messages that are older than any specific date

Find items large than: The user can discover and remove email messages that are larger than any specific size

Fix 3: Move Folders from OST to PST: It is also called archiving of older email folders. Follow the below steps:

Outlook 2013 Version:

• Browse File
• Go to Cleanup Tools
• Then press Archive

Users of Outlook 2016/ 2019 version follow the below steps:

• Browse File
• Click Info
• Then go to Tools > Clean up old items
• Choose the second dropdown option reading “Archive this folder…..subfolders’
• Select an email folder from the option to Archive
• After this provide the date in the ‘Archive …..older than:’
• Press Browse to choose a save location. Press OK

How to Convert OST to PST Safely?

As discussed earlier, the best strategy is to convert an Outlook data file (.ost) to PST format. The easiest way to perform this task is to utilize some professional converter tool. Such tools are known for their quick conversion speed. They are also independent. It means they can perform this task even without the availability of the Exchange Server. Tool like Stellar Converter for OST is known for its ability to handle an inaccessible OST of any size. This software can also recover deleted mailbox items. It also provides filtering option for selected recovery. IT admins applaud its capability to extract mailbox data with full precision and accuracy. It also maintains the folder structure and hierarchy.

Conclusion:

OST is a crucial element of Outlook. Its space optimization is essential to ensure smooth operations of this email client. A large OST file is a serious concern. In the above guide, we have learned the reasons as well as the consequences of an oversized data file (.ost). We have also learned 3 manual ways to fix the issues. But all these methods involve complex steps. They also pose a threat to the data stored in the OST. The best way is to convert OST to PST. It is a long-term and reliable solution. It improves performance besides ensuring data safety. The easiest way to perform such conversion is by utilizing a software like Stellar Converter for OST. It features advanced filtering option for selective recovery. This tool can recover data from an inaccessible OST as well. It has the capability to convert even a large data file.