Outlook is using an old copy of your Outlook data file [Fix]

If you open MS Outlook and see an error message, Outlook is using an old copy of your Outlook data file (.ost), then use ...
Houston Chronicle

Opening an OST File in Microsoft Outlook

Microsoft Outlook categorizes Outlook Data Files into two types: PST and OST files. PST files contain data stored in your personal folders, whereas OST files contain data downloaded off your Exchange ...
Ars Technica

[ OUTLOOK ] Won't startup w/o OST, but can't create OST?

Go into the mail profile and tell it not to use Offline Mail Folders. That's what the OST file is.
Your Story

Convert Outlook OST to PST Manually or using a software

As we all know that Outlook is one of the most popular desktop email clients, and provide high security & easy interface. The Outlook saves all the data in the local machine with two file extension ...