I Want to suggest you use the SysInspire OST to PST File converter software for better experience you can go with and It provides you free demo version also to save the 25 items from each and every folder of the OST mailbox. This software includes some formats to convert the OST file to PST,EML,EMLX,MSG,MBOX,HTML,vCard and office 365 and it also splits the bulk file into multiple files. It allows 24/7 and 30 days technical support to the users.
eSoftTools OST to PST converter software is the perfect utility for you. It's able to migrate Outlook OST files to PST and many other file formats such as EML, EMLX, HTML, MSG, MBOX, ICS, vCard, Zimbra, Office 365, Gmail, and Yahoo. It also gives you the opportunity to convert multiple Outlook OST files at the same moment with the bulk conversion option. It also gives you a 30-day money-back guarantee and strong customer care support. Check the free demo version of this tool from the official website of eSoftTools.