As we all know that there are so many solutions available like manual or third-party tools. So it's too complicated to select the right way to migrate the data from the Outlook PST to Office 365 Exchange Online mailbox. But to make your decision easy and simple I recommend the A2Z Migrations for PST to Office 365 tool. This is an advance way to import the data from the Outlook PST to o365 Primary or In-Place Archive Mailbox at a time or Parallelly.

More Info- http://www.a2zmigrations.com/pst-to-office365/