Opt KDETools ost to pst converter free software is wonderful tool that converts multiple Offline OST to PST, Zimbra, Gmail, EML, EMLX, vCard, ICS, MSG, Office 365, Live Exchange server, MBOX and etc.. You can export single as well as multiple OST files to PST without disturbing the internal folder hierarchy. It has the capability to transfer large size OST file and split them during the conversion process. It supports all editions of MS Outlook up to 2019.
Manual Method to Import OST file into Outlook
Click Properties and then go to Email tab and select the Exchange account and click on Change.
If “Use Cached Exchange Mode” checkbox is selected then uncheck it.
Now click on More Settings and then go to Advanced tab, and click Offline Folder File Settings.
More info- ost to pst online