Outlook OST File Too Large: How to Reduce Size and Improve Speed

An oversized OST file can cause Outlook to run slowly, freeze, or fail to sync with your Exchange server. This offline data file grows as you receive emails and attachments, storing a local copy of your mailbox. This guide provides clear steps to reduce the OST file size and restore Outlook’s performance.

Why Your Outlook OST File Grows Uncontrollably

The OST file is your local cache for a Microsoft Exchange or Microsoft 365 mailbox. It allows you to work offline and sync changes later. Its size increases with every email, calendar item, and attachment saved in your mailbox. Without management, it can reach tens of gigabytes.

Large files slow down Outlook because the program must read and write more data to your hard drive. Performance issues like lagging, crashes, or sync errors often start when the OST file exceeds a healthy size. Regular maintenance is required to keep it manageable.

How Cached Exchange Mode Affects Size

Cached Exchange Mode is the primary feature that uses the OST file. The sync slider in your account settings determines your local cache’s timeframe. A setting of “All” downloads your entire mailbox history, which can create a very large OST file quickly.

Steps to Reduce Your Outlook OST File Size

Follow these methods in order to effectively shrink your OST file and improve Outlook speed.

Method 1: Use the Mailbox Cleanup Tool

1. Open the Cleanup Tool
   In Outlook, go to File > Tools > Mailbox Cleanup. This opens a dialog with several size management options.
2. View Mailbox Size
   Click the ‘View Mailbox Size’ button. Review the size of each folder, like Inbox and Sent Items, to identify the largest ones.
3. Find Old or Large Items
   Use the ‘Find items older than’ or ‘Find items larger than’ tools. Select a date or size, then click Find to see matching messages.
4. Delete or Archive Items
   Select the items found and delete them. For long-term storage, move them to a local Outlook Data File by clicking Archive on the File tab.
5. Empty the Deleted Items Folder
   Right-click the Deleted Items folder and select ‘Empty Folder’. This permanently removes items and allows space to be reclaimed.

Method 2: Adjust Sync Settings

1. Open Account Settings
   Go to File > Account Settings > Account Settings. Select your Exchange or Microsoft 365 account and click Change.
2. Change the Mail to Keep Offline
   Click ‘More Settings’, then go to the Advanced tab. Find the ‘Mail to keep offline’ slider.
3. Select a Shorter Timeframe
   Move the slider from ‘All’ to a shorter period like 12 months or 3 months. Click OK, then Next and Finish.
4. Restart Outlook
   Close and reopen Outlook. It will begin removing older data from the local OST file based on your new setting.

Method 3: Compact the OST File

1. Enable Automatic Compaction
   Go to File > Options > Advanced. Under Outlook start and exit, check the box for ‘Compact all Outlook data files when Outlook closes’. Click OK.
2. Close Outlook to Trigger Compaction
   Exit Outlook completely. The compaction process runs in the background and may take several minutes for a large file.
3. Verify the New File Size
   Reopen Outlook. To check the new size, go to File > Account Settings > Account Settings, select the Data Files tab, and note the size listed for your OST file.

If Your OST File Remains Large After Cleaning

Outlook Still Shows Large OST File After Deletion

Deleted items are only marked for removal until you compact the file. Ensure the automatic compaction setting is enabled and that you close Outlook completely. You can also manually locate and delete the OST file while Outlook is closed, but Outlook will download a new one on startup.

Sync Slider Grayed Out or Unavailable

This usually means your Exchange administrator has set a policy. Contact your IT department to request a change to your cached mailbox settings or to increase your mailbox quota on the server.

Outlook Crashes When Accessing Large Folders

Extremely large individual folders can cause instability. Use the Mailbox Cleanup Tool to find large items and move them to a local PST archive file. This reduces the load on the OST cache.

OST File Management: Proactive vs Reactive Approaches

Item	Proactive Management	Reactive Fix
Primary Goal	Prevent the OST from growing too large	Shrink an already oversized file
Key Action	Set Sync Slider to 12 months or less	Run Mailbox Cleanup and compact the file
User Effort	Low, one-time configuration	High, manual cleaning and verification
Outlook Downtime	None	Possible during compaction
Best For	New accounts or preventing future issues	Restoring performance on a slow mailbox

You can now control your Outlook OST file size using the built-in cleanup and sync tools. Setting the sync slider to a shorter period is the most effective long-term prevention method. For advanced management, use Outlook’s AutoArchive feature to automatically move old items to a separate PST file on a schedule.