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If you want to know step by step OST to PST conversion process, then I would suggest you download a free trial version of Shoviv OST to PST Converter software to know its features and working steps.
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(i) Open MS Outlook application, Select File option.
(ii) Choose ‘Archive’ and choose the folder(s) or file(s) that needs to be archived.
(iii) Browse the location to save the Archive file(s) or folder(s).
(iv) Then, click the Finish, to complete the process.