Transition from Office 365 to Office Pro 2021

I have used MS Office on my desktop for many years.  I switched from a standalone version to Office 365 on my desktop a couple of years ago.  I have already purchased MS Office Pro 2021 and I’m getting ready to switch back to the standalone version.

I have two email accounts setup in Outlook, and therefore I have two ost files (they used to be pst files).  I assume there is one ost file per email account. One of the ost files is older and much larger than the other one.  I also access my two email accounts online at Outlook.com and the emails, folders, contacts, and calendars seem to stay synced automatically with my desktop Office 365.

I sometimes, but very rarely, use Excel and PowerPoint.

I’m not sure how to transition from O365 to Pro 2021. Do I just install the new Pro 2021 and it sees the O365 and transfers the ost files automatically?  Will it guide me in the process? Will I need to uninstall (delete) O365 when the transfer is complete?  Will all my emails and folders and contacts and calendars be in the new standalone Pro 2021 after I install it?

Since one of the ost files is many years old, when I switch back to the standalone version, should I start with a new ost file?  It seems like that would get rid of any old crap left over from many years of use.  Or would that be a bad idea? I really don’t have any idea what’s in an ost file. 

Thank you in advance for help you can give me before I start this process.