OST is Exchange server offline files, due to corruption, tansfer file to another system, corruption and maintenance in exchange server are main reasons exist, cause behind conversion of OST file into PST. Outlook PST files are easily accessible and manually can be obtained by conversion. Mainly there are two popular methods -
1.Archiving OST data to PST file
2.Using export feature in Outlook
For detailed knowledge about this conversion method, users can visit here : 
https://www.datarepairtools.com/blog...file-manually/