Hi Coolemail,
The article below is our Official documents related to use Outlook 2010 to import PST file.
Import Outlook items from an Outlook Data File (. PST)
Moreover, based on my experience, here is another way for you reference. We can open the PST file easily in Outlook, then we need to move the emails or folders manually to the Office 365 account. I also tested it on my side and it worked. For your specific situation, you can have a try and check to see if it would be easier to move the large PST file on your side.
For more information, please refer to this link: Open, change, or close Outlook data files
Thanks,
Tiny Duan