Basically, the OST file cannot be shared that's why the OST is converted to PST. Then the PST file is shared but the problem arises with how to import PST files in Outlook web. You can do this just by performing the several steps.
1. Download and set the tool by Outlook as the Source and Office 365 as the destination.
2. Select the data as per the date.
3. Enter the source and destination details.
4. Select users and start importing.
