where can i find my old .ost file

To locate and remove the old .ost file and keep using classic Outlook, follow these steps.

1. Close Outlook completely (both classic and new Outlook).
2. Find the .ost file on the computer:
   1. Open File Explorer.
   2. In the address bar, paste one of these paths and press Enter:
      • %localappdata%\Microsoft\Outlook This expands to something like:
      • C:\Users\<YourUserName>\AppData\Local\Microsoft\Outlook
   3. In that folder, look for files with the .ost extension (for example, ******@org.com.ost, Outlook.ost, Outlook0.ost, etc.). These are the offline Outlook Data Files.
3. Delete or rename the problematic .ost file:
   • If Outlook is telling that a specific .ost is old or corrupted, delete that .ost file from this folder.
   • Alternatively, rename it (for example, change .ost to .old). Outlook will create a fresh .ost the next time it starts and will re-download data from the server.
   This is the recommended fix when errors are detected in an .ost file.
4. Start classic Outlook again:
   • Open classic Outlook for Windows. It will recreate a new .ost file in the same folder and download mailbox data from the server.
5. Ensure classic Outlook is used instead of new Outlook:
   • If both classic and new Outlook are installed, open classic Outlook explicitly from the Start menu (look for the icon labeled “Outlook” or “Outlook (classic)” depending on the build).
   • If new Outlook keeps opening from a shortcut, adjust the shortcut to point to classic Outlook or pin classic Outlook to the taskbar and use that icon.

If Outlook still complains about an old .ost after these steps, repeat step 2 and remove any remaining .ost files associated with that profile so Outlook can rebuild them.

References:

• Errors have been detected in your .ost file in Outlook
• Synchronization problems occur in Outlook and in Outlook Web App
• What if Outlook can not open after the failure of transferring Outlook related files?
• You can't change the location of the offline Outlook Data File (.ost) in Microsoft Outlook
• Open and close Outlook Data Files (.pst)
• You can't open classic Outlook on a new Windows PC
• How do I recover my old email messages, my contacts and everything else I had in Outlook? - Microsoft Q&A
• Why does Microsoft keep switching me from Outlook Classic to New Outlook when I do not have that option enabled? - Microsoft Q&A
• What to do when Outlook will not open on my PC? - Microsoft Q&A
• Class outlook is not working properly after Windows 11 version 25H2 update - Microsoft Q&A