You can visit here; http://itknowledgeexchange.techtarget.com/itanswers/can-i-convert-ost-to-pst-file/
All manual solutions are suggested here. But do you know Import/export option of Microsoft Outlook will help you to save OST as PST only if there is connectivity between Outlook account and Exchange Server.
In absence of Exchange Server and original MAPI profile you should relay on free demo of any reliable OST to PST Software. This type of free demo version of tool does not demand money and any type of investment from your pocket as these types of tools are completely free.
I would like to suggest you to use one free demo tool among availability of various market’s tools. I have seen that it exports 25 ost emails into PST just in blink of an eye. Yupppp it’s completely free.
You can also download free trial run of software here; http://www.ostpstsoftware.com