You can use a manual way to transfer emails from OST to PST in Outlook if you want to know how the manual process will be then you need to follow these steps.
Step 1: Launch the Microsoft Outlook email client.
Verify that your Outlook account is connected to the OST file.
The data must be saved in Outlook as a PST file.
Step 2: Select "File" from the menu.
When Outlook is open, click the File tab in the window's upper left corner.
Of the options, you can select Open & Export.
Next, select Import/Export.
Step 3: "Export to a File".
In the Import and Export Wizard, select Export to a file.
Click "Next."
Step 4: Select an Outlook data file (.pst) format.
On the following screen, select Outlook Data File (.pst).
Click "Next."
Step 5: Select the export destination folder.
Select the location or email address you wish to send the file to.
You can select specific email sections, such as Sent Items, Contacts, or Inbox. If you wish to add subfolders, make sure the add subfolders box is picked.
Step 6: Choose a location to save the PST file.
After selecting where to save the PST file, click Browse.
After naming the file and selecting a location on your computer to save it, click Finish.
Apart from that users have one more solution with TrustVare OST to PST Converter Software which is the best solution and supports the bulk OST file conversion solution without a single error. This software has a simple and easy graphic user interface so that users can easily operate it without a lack of technical understanding. Users can install it on windows OS to transfer emails from OST to PST in Outlook and users will get a free demo version to examine its features.