If the mailbox is still connected, the easiest route is inside Outlook itself, use the import and export option to export the mailbox to a PST file, that keeps the folders and mail intact.
Just renaming the file does not work, an OST is tied to its original mailbox and profile, so an orphaned OST from a deleted account will not open on its own. In that case you need a converter that can read the OST directly, and it is worth testing it on a copy first to check the folders and attachments come across properly.