Step 1. Open Outlook and select File.
Step 2. Select the Open & Export option.
Step 3. Click on Import / Export.
Step 4. Select the option, Import from another program or file, and click Next.
Step 5. Select the option, Outlook Data file (.pst), and click Next.
Step 6. Select the Browse option and chose the PST file you wish to import.
Step 7. Select the option, Replace duplicates with items imported. Click Next.
Step 8. Under Select the folder to import from option, expand the Outlook data file folder, and select Contacts (using Filter). Check the box, Import items into the same folder, and type in the Office 365 email address you want the contacts to be imported to. Click Finish.
Step 9. You will then see the progress box of importing the contacts from PST to the Office 365 mailbox. The process will automatically come to an end once the transfer has been done.