Download the Recovery Toolbox utility software at the following link: http://www.recoverytoolbox.com/osttopst.html#p1.

Open the Recovery Toolbox utility and click on the "Search for .pst & .ost files" button.

Choose the disk or folder containing the .ost files that you wish to convert, and then click the "Search Now" button. Locate and select all the files with .ost extensions that you wish to convert.

Select the folder and location in which the converted files will be placed.

Choose the first option of saving to the disk as Microsoft Outlook storage. Then click the "Save" button, and you are finished. The utility will convert the files and save them in the location you specified.