Is it OK to do it that way?

This was not included in the message above for some reason. 'Currently I copy the PST file to an external disc and restore it after Macrium has finished the restore'

I don't use Outlook, but I think the general idea was to keep Outlook closed during backups if I remember reading past threads correctly.

I’ve seen multiple threads where users reported issues with PST files that were backed up while Outlook was open. That surprises me since I would expect Microsoft of all companies to have a VSS writer for Outlook to mitigate that — and maybe they do in newer versions? — but if it were me, I would play it safe there.

It will backup fine when done.  However, if it is opened again after the backup then the .OST (at least in O365 commercial they use an .OST file) will become aged and if restored to this older version Outlook will complain that it is an older version and download all the contents from the cloud again. 

If you are using some other .PST files, perhaps for archiving, then as long as Outlook is closed it will backup and restore fine.  I do mine all the time, I also use a synch tool to copy my archive .PST files off to various encrypted disks for additional backups.  I copy my .PST file to another PC as well, to insure that this archive .PST is kept up to date. 

In short, as long as Outlook is closed it will backup the mail files.  If you are concerned about the main. OST file, it will back it up too, but the backed up file becomes out of date immediately as soon as you restart Outlook and restoring just it (or the whole OS) will prompt Outlook to download and update the .OST file.  You are already getting a backup of the.OST in the MS O365 cloud.    

The OP asked about PST backup but I have MS 365 Personal, not Business, and it also uses .OST for Outlook. As Rootman said, if the OST is out of date after the restore, it gets updated. In effect, the Outlook server contains an easily accessible backup. I consider the 365 a good deal for me since I get all the latest versions of the Office apps, Outlook and 1TB of cloud for about $70US per year which is very competitive for just the 1TB cloud alone without the Office.

I wouldn’t say that the Outlook server contains a backup. I would say that the OST is an offline cache of the Outlook server that represents the authoritative source.

I think it’s dangerous to think that way. Your emails are already protected against a disk failure as a consequence of being stored in the cloud. An offline cache file is not a backup of the cloud, since something like a malicious deletion in the cloud due to compromised credentials would be replicated down to the cache — if the data was cached there to begin with. I realize you have Reflect backups, but backing up a cache file is also an odd way to go about things in my opinion. I guess it’s better than nothing, but this is why at least for businesses, there are solutions that actually back up cloud mailboxes directly.

Yes, most Reflect users probably are protecting against a disk failure. But if the disk doesn’t even have the authoritative copy — or indeed an “equal” copy — then I would argue that that’s not something most people use Reflect for.

I get what you are saying, @jphughan, but it appears I can download all the Outlook email to the PC (right now it is set for 1 year) such that it would a have a copy of what is on the Outlook server so backing up the OST file in an image or elsewhere should provide a backup for that point in time. Of course, I'm saying this without having tried it since it is not really a worry for me being a personal, rather than business, user and I always save anything of importance to a folder outside of email.
If I were using my PC to run a business then I'd certainly have a more rigorous approach to the problem.