Use the "Import/Export" feature in Outlook:

1. Open Outlook. 
2. Go to File > Open & Export > Import/Export. 
3. Select "Export to a file" and click Next. 
4. Choose "Outlook Data File (.pst)" and click Next. 
5. Select the OST file folder to export and finish.

Note: This method requires access to the original Outlook account linked to the OST file. If you think that this method is little bit complex or time consuming for converting your large volume of OST file then you can take help of IT professional recommended EmailsGuru OST to PST Converter.