OST file is an Outlook data file that is created when Outlook in Configured with Exchange Server having Cache Exchange Mode enabled. That OST file stores the entire precious content including emails, tasks, calendars and contacts etc. if you are in exchange server everything will work fine. The problem will only arise if you need to access that data without Exchange Server which is not possible without taking the help of any reputed third party application.
Based on my personal experience in OST to PST Conversion I would like to recommend MailsDaddy OST to PST Converter here. The application is really a great way of converting your Outlook OST files to PST file format. The tool has several options for converting your OST file. You can convert your OST file in PST, EML, MSG, MHTML and MBOX file format. The application also has additional Default option with which users can convert Emails in MSG, Contacts in VCF and Calendars in ICS file format.
The application also has several other useful features some of them are discussed below:
1). Easily Convert your OST file to PST file format.
2). Convert OST to MBOX, EML, MSG, MHTML file format.
3). No Outlook and Exchange Server need for Conversion.
4). Compatible with all Windows Operating Systems.
For More Information: http://www.mailsdaddy.com/ost-to-pst-converter/