Fix OneDrive and Outlook “Not Responding” with Cache Reset and PST Cleanup
This guide outlines how to diagnose and resolve these performance issues using Microsoft-recommended practices.
🔍 Symptoms
⚠️ What You May Experience
- OneDrive stuck on loading screen
- Outlook becomes unresponsive intermittently
- High CPU or disk usage during sync
- Frequent delays while opening Outlook or accessing the mailbox
🧠 Root Cause
Three common causes working together
- PST files in OneDrive: Outlook PST files are not supported in OneDrive sync. Due to file versioning, PST files continuously change and trigger excessive sync operations, causing performance degradation.
- Corrupted OneDrive and Outlook cache: Cached files in local profiles can become inconsistent, causing apps to hang.
- Invalid folder naming conventions: Special characters in OneDrive folder names may disrupt synchronisation processes.
🛠️ Step-by-Step Resolution
Remove Unsupported Files from OneDrive (PST Cleanup)
Identify and remove PST files and any large, constantly changing files from the OneDrive sync folder.
✅ File Locations
- OneDrive folder:
C:\Users\<username>\OneDrive - Move PST files to local storage:
C:\Users\<username>\Documents\Outlook Files - Also remove: backups, VM images, or any other large constantly-changing files
Reset OneDrive Cache
Run the OneDrive reset command to clear the local cache and force a clean re-sync.
# Step 1 — Reset OneDrive (Win+R, then paste this) %localappdata%\Microsoft\OneDrive\onedrive.exe /reset # Step 2 — If OneDrive doesn't restart automatically, run this %localappdata%\Microsoft\OneDrive\onedrive.exe
Reset Outlook Cache (Clear the OST File)
Close Outlook, then delete the OST file — it will rebuild automatically on next launch.
✅ OST File Location
- Navigate to:
C:\Users\<username>\AppData\Local\Microsoft\Outlook\ - Delete the
.ostfile - Reopen Outlook — it will rebuild the cache from Exchange Online automatically
⚠️ Note
AppData is a hidden folder. Enable Show hidden items in File Explorer (View → Hidden items) before navigating to this path.
Verify OneDrive Folder Naming Standards
Ensure folder names do not contain special characters that disrupt synchronisation.
❌ Characters Not Allowed in OneDrive Folder Names
Avoid these in any file or folder name synced via OneDrive:
” * : < > ? / \ | #
✅ Good example: Project_Documents_2026
Validate OneDrive Sync Health via PowerShell
Check OneDrive site status and storage usage across the tenant using SharePoint Online PowerShell.
# Install and connect to SharePoint Online module Install-Module Microsoft.Online.SharePoint.PowerShell -Scope CurrentUser Connect-SPOService -Url https://yourtenant-admin.sharepoint.com # Check OneDrive site status and storage usage Get-SPOSite -IncludePersonalSite $true -Limit All | Where-Object { $_.Url -like "*-my.sharepoint.com/personal/*" } | Select Url, StorageUsageCurrent
Check Mailbox Health via Exchange Online PowerShell
Large mailbox sizes can also contribute to Outlook lag — check user mailbox statistics.
Connect-ExchangeOnline Get-MailboxStatistics -Identity "user@yourdomain.com" | Select DisplayName, TotalItemSize, ItemCount
💡 Best Practices & Recommendations
- Do not store PST files in OneDrive — use Exchange Online Archiving or local storage instead
- Regularly clear client caches — particularly for users experiencing sync or performance issues
- Adopt proper naming conventions — avoid special characters to ensure smooth syncing
- Use Exchange Online mailboxes instead of PST — provides better performance, backup, and compliance capabilities
💡 If This Affects Multiple Users
Widespread issues may indicate broader policy or storage management gaps. Consider implementing tenant-wide controls to prevent PST usage in OneDrive via SharePoint Online and OneDrive admin policies.
📚 References & Further Reading
- 🔗 Learn about Importing PST Files to Microsoft 365 — Microsoft Learn
- 🔗 Connect to Exchange Online PowerShell — Microsoft Learn