Microsoft Outlook itself equipped with import/export options. You can also use the same, check below listed steps:

1. Launch Outlook and click on File from Menu Bar
2. Click on Open & Export and Then Select Import/Export
3. Select Export to an Outlook Data File .pst
4. Select Profile and Browse Location and Save Exported File

Note: - But in case of Orphaned OST, Exchange Profile Not Available, Issue in Server Connectivity, Another Users' OST File. You cannot perform the above-listed steps. To deal with such situations, follow the below-listed steps:

1. Visit the Official Site to Download OST to PST Converter Software 
2. Launch The Tool and Browse OST File Which You Want to Convert
3. Preview Complete Data into Software Panel in 8 Different Modes 
4. Select PST Export Option from the List of 15 Export Options and Save

The free demo version of this tool is also available.