Import OST as offline files into Outlook 365

[Ffmmuc]

I need to set up a new laptop with a new Microsoft ID. I also have my old company laptop with thousands of emails. Unfortunately, my company went bankrupt, so the mailboxes are now offline and can't be synced anymore. We can neither receive, nor send any new mails. But the offline emails are stored in Outlook 365 on the old laptop and I can still open and read them.

I need to save these emails and transfer them to my new laptop. When I export them as a PST file, it is 1.3 GB, so there is data inside. But when I import it into my new Outlook, no emails show up, only a few thousand sync errors. I think this happens because Outlook tries to connect to the Exchange server when opening the PST file.

I read that OST files are the offline version of the emails. How can I get this offline data into my new Outlook? I saved the OST files on a USB stick, but how do I import them? Converting to PST and then importing doesn't seem to work.

Both laptops are running Windows 10 with Office 365.