Quick answer: An Outlook OST file is a local mailbox cache. Do not delete it blindly while Outlook is open; reduce cache settings or recreate the profile safely.
What this means
Outlook can store gigabytes of mail locally in AppData. This is common on small SSDs, especially with large mailboxes and long offline cache windows.
Safe cleanup steps
- Close Outlook before changing or cleaning mailbox cache files.
- Check the OST file size under AppData Local Microsoft Outlook.
- Reduce Cached Exchange Mode mail to a shorter period if available.
- Archive or clean large mailbox attachments from Outlook.
- Let Outlook recreate the OST only after confirming mail is synced to the server.
What not to delete
Do not delete PST archive files unless you have a backup. Do not delete an OST while Outlook is running.
When to use DiskTidy
Use DiskTidy when you want a local, review-first scan for temporary files, browser cache, cleanup candidates, and large files before deciding what to remove.
FAQ
Will cleanup make Windows faster?
It can help when the C drive is nearly full, but cleanup is mainly about restoring free space and update reliability.
Should I use one-click cleanup?
Use review-first cleanup. One-click deletion is risky if it includes Downloads, personal folders, browser profiles, or system components.