No confirmation received, but we're (almost) sure no hidden option exists to import PST content from Outlook 2016 for Mac into you O365 mailbox :-). Managed a workaround using a Windows device with Outlook 2016 instead.
Be aware that the import goes (pretty) fast, because items are loaded into you local cache. Then it will take aconsiderable time, even hours depending on the amount data, to get the local items synchronized into the online O365 mailbox.
Check Outlook's Synchronization Status after a while, but also be aware that Outlook initially states "All folders are up-to-date", only after while the status will change to synchronizing. Give it time :-). If you restart Oulook in between give it time to see Synchronization Status changing, even when it mentions you are "Connected to: Microsoft Exchange".
If you initially tried to import the PST using Outlook 2016 for MAC, remove this local instance "On My Computer", to ensure this stail version of items is not accidentally used to send and/or receive items.