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Both OST and PST files hold importance to an Outlook user. They function as storage containers of all the user’s data including emails, contacts, tasks, calendars, etc. The only difference is that PST and OST store this information in the online and offline modes, respectively. Due to the vast quantity of information, the file size turns out to be oversized. It gets tricky to manage oversized OST files and in addition, corruption in any of these will result in Outlook data to suffer. To be prepared in advance for such possible future scenarios, the OST file size limit has to be your highest priority. Hence, reducing the OST file size and solving one-way, oversized OST file issues in Outlook is crucial. Also, the file’s storage capacity depends on the MS Outlook application’s installed version.

Outlook Version OST File Size Limit
Outlook 2002 and earlier versions 2GB
Outlook 2007 20GB
Outlook 2010 and later versions 50GB
Instant Solution

Try automated tool Kernel for OST to PST Converter to resolve OST file corruption issues due to Oversized OST file, Virus Infection, Storage Media Failure or any other reasons.

How do OST files get oversized?

OST file contains all the data you have generated or received since the account is activated on Outlook. It includes all emails (different folders), attachments, calendar schedules, tasks, contacts, notes, hidden items, public folders, etc. When you forget to delete unimportant data, it gets piled up, exceeds the file storage limit, and causes corruption in the OST file and troubles in Outlook. Large-sized attachments also cause oversized OST files. So, if you don’t prioritize keeping the OST file size within the limit, potential difficulties may await you.

Issues in Outlook due to oversized OST file:
Manual solutions to fix errors because of “Oversized” OST files

Basic Activities: Follow these primary activities or instructions to manually eliminate the unused and unwanted data from the OST file.

  • Delete unwanted emails from your OST folders permanently
  • Save crucial and large-sized OST file attachments on your system
  • Ensure the clearing of the Deleted Items folder in the OST mailbox is cleared
  • Rename your OST file from the default location and let it be generated once again.

Disabling all add-ins: Add-Ins in Outlook may disrupt or affect its normal functioning. To improve the performance of Outlook, disabling Add-Ons is a suitable option. To perform this, follow below steps –

  1. In Outlook, go to the File menu, select Options, then click Add-Ins in File menu
  2. Under Manage option, select COM Add-Ins, and click COM Add-Ins
  3. Unselect all the Add-Ins that are currently not in use or non-functional and click OK.Unselect all the Add-Ins

Archiving OST file data: Archiving OST file data might reduce OST file size and clear some space occupied on the server. You can change the archive OST file location in your system. Archiving an OST file may prevent your OST file from crossing a predefined size limit. To archive the file, follow the steps shared below.

  1. Click on the File tab>click Cleanup Tools>select Mailbox Mailbox Cleanup
  2. Next, two options are available:
    • You can move older items in the mailbox to an archive folder by clicking on the AutoArchive option.
    • Click Empty and delete items from the Deleted items folder.

Compressing Outlook data file: Shrinking of OST file size can become a reality using the In-built utility in Outlook. Reducing file size might help improve Outlook performance provided the user has deleted substantial items. To reduce the OST file size, follow the given steps.

  1. Delete those items which are not required further.
  2. Open the folder list view, and in the Navigation pane, right click on the folder, click Deleted items and click Empty folder.
  3. In Outlook, click on the File tab and from Account Settings, select Account Settings.
  4. Click on the Data Files tab, select the data file that requires to get the file size reduced, and click the data file to be compacted
  5. In the opened dialogue box, click the Advanced tab and then click Outlook Data File Settings.Outlook Data File Settings
  6. In Outlook Data File Settings dialogue box, click Compact Now, which is the final step.Compact Now

Using clean-up option: As a means of reducing the size & having a compact OST file, it is possible to delete surplus emails and other items. Outlook provides a clean-up option for removing emails from the Deleted Items folder. Follow these steps to do so:

  • Access Outlook & navigate to the File tab.
  • Go to Tools and tap on Mailbox Cleanup.
  • Click on Empty under the “Emptying the deleted items folder permanently delete those items”.

Disable shared folder synchronization: A shared folder can also rapidly increase OST file size and cause oversized OST file problems and other Outlook performance problems. By following these steps, you can disable synchronization of Shared Folders:

  • Go to the File menu and navigate to Account Settings > Account Settings.
  • From the tab Data Files, choose your OST file.choose OST file
  • In Settings, go to Advanced and select Outlook Data File Settings.
  • Unmark the option Download Shared Folders and hit OK.Download Shared Folders
  • Restart the Outlook application to finish.

The above-discussed methods might help reduce OST file size or clear space on the server by moving these files to a different location. But if OST files become corrupt due to their size, you might lose their data. A third-party tool comes in handy to overcome such oversized OST file issues if it has caused inaccessibility issues in Outlook.

Kernel for OST to PST Converter is a useful utility that repairs OST files completely and converts them into PST files efficiently without losing any data. You can download “Trial Version” for free and retrieve your lost or inaccessible data in different formats as per the requirement. Features like In-built search option, smart filters, splitting OST files, a complete preview of file and File Analysis report can be utilized through this utility. The user can make use of another remarkable feature Saving snapshot of the current process for future use by buying a “Full Version” of the utility.

With the ‘split PST’ feature of the software, the user can resolve issues related to oversized OST files. The tool converts oversized OST file size to PST format and then, using Split PST feature, saves the file into multiple PST files. Let us understand it with the steps.

  1. Launch the tool.
  2. On the Home Screen, click Browse to add the oversized OST file from the system location. Click Next.
    add the oversized OST file
  3. Preview the retrieved OST mailbox data and click Save below the menu bar.
    Preview the retrieved OST
  4. Select PST File (MS Outlook) file from the list and click Next.
    Select PST File
  5. Apply filters as per your choice and click Next.
    Apply filters
  6. Use Split option by selecting the splitting file size against it as per your requirements. Click Next.
    Use Split option
  7. Click Browse and provide the saving path for the PST files on your system. Click Finish.
    provide the saving path for the PST files
  8. The saving process will start and finish soon.

Hence, dealing with oversized or corrupted OST files with this converter tool is quite easy.


Outlook and Exchange users suffer from issues, including data accessibility and synchronization, due to an oversized OST file. The blog suggests filtered practical DIY solutions like disabling add-ins, archiving OST files, compressing OST files, etc. For dealing with over-sized OST files, reliable OST to PST Converter software is recommended.

Proposed by Mr. Simon_Wu Microsoft contingent staff, Moderator

Moderator answer

Read more:

Q- What causes OST files to be large?

A- Outlook Offline Data (OST) files are a vital component of the Outlook application, as they store all mailbox data for users locally on their computers. Typically, the mailbox items make up much of the file size, but there are other items, such as public folders & hidden items, which may contribute to an increased file size. It is crucial to manage these items effectively to ensure that the OST file remains at a manageable size & prevents any potential productivity issues.

Q- Can I Delete old OST files?

A- It is entirely possible to delete old OST files without any significant repercussions. This action will not result in any data loss as the said file can be fetched back from the Exchange Server by downloading again when required. So, if you have any old OST files hanging around, taking up valuable space, you can delete it. 

Q- How do I clear my OST cache?

A- To clear the OST cache follow the below steps:

  • To begin the process, it’s necessary to exit Outlook.
  •  Next, click on the Start menu & choose Run. 
  • A window will appear, & in the Open box, type “%LOCALAPPDATA%\Microsoft\Outlook\” and press OK. 
  • Once you locate the .ost file, you will need to rename it. 
  • If you cannot see the file, be sure to activate the Show Hidden Files and Show Extensions features. 
  • Finally, restart the computer to complete the process.

Kernel for OST to PST
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