There are two methods to convert OST files into PST files-:
1. Manually Conversion
2. Software Conversion
First manually type of conversion simply need a good knowleadge about technical things. so it is hactic and time taking for most of the users. But if we talk about the second type of conversion then it is totally effiecient and it doesn't take a lot of time. Now the thing is it is paid or free so i suggest you a tool of Aryson Technologies which is totally free for 50 OST files. Aryson OST to PST converter take fw seconds to convert OST files to any PST files and store it into your system.