This has happened twice now. I removed the profile in Outlook and created a new one and everything worked fine for about a week. This morning Outlook failed to open again after the splash screen with the message 'ost. in use'. Has anyone seen this issue before or knows of the solution.

Pierre McCloud

Hi Pierre,

Usually, this occurs due to another process or program is holding the file, for example, a third party add-in, anti-virus software and etc.

To troubleshoot the issue, I would suggest we try following steps:

1. Start Outlook in safe mode. If Outlook can be launched in safe mode, then it's telling us that an Add In is causing the issue, we shall troubleshoot your Outlook add-ins in this case: Press Win + R and type "outlook.exe /safe" in the blank box, then press Enter.
2. If you have any anti-virus software installed in your machine or integrated with Outlook client, please temporarily disable it and then try again.
3. Do you have Microsoft Lync installed on the same machine? Microsoft Lync also accesses your .ost file, so if you have Lync client running on the machine, try to exit it and try to open Outlook again.

Hope this helps. Please feel free to post back with any findings.

Regards,

Ethan Hua
TechNet Community Support