You can convert Microsoft Exchange OST file in PST file free of cost, just follow the following source.
www.softwareimperial.com/free-download.html
There are some expert software also available like Microsoft for OST to PST Conversion with help you. By this 3rd party software, one can easily convert damaged Exchange OST file in PST file format. You can also checkout the software with a free trial from here www.osttopst-software.com