I recently allowed OneDrive to back up my documents to the Cloud.
After the uploading had finished to the Cloud, I got an error when I tried to open and download my emails using the Outlook mail client.
The error was “Outlook .PST cannot be opened”
For some strange reason, MS chooses to create the “Outlook Files” folder in the Documents folder, instead of the root of the drive.

OneDrive does not back up the root of the drive, it only backs up your Documents; your photos and your desktop folders to the Cloud.
Whilst trying to troubleshoot my problem, I saw that the path to my .PST file was now in OneDrive. Microsoft does not recommend putting your .PST file in the Cloud.
As part of the synching process, OneDrive locks the .PST so that the Outlook programme can’t access the .PST file.
The only way to resolve this is to move the PST file from the Cloud back onto your computer but not in the Documents folder.

Firstly close Outlook!
As a back up, use a USB stick to back up the Outlook Files folder in case something goes wrong during the next steps.
Once you have backed up your Outlook Files folder, then I suggest that you simply move the Outlook Files folder to the root (C:) of your drive from the My Documents folder.
After you do that and you open Outlook, you will get an error that Outlook can’t find your .PST.
Point Outlook to the .PST in the Outlook Files folder on your C drive root, that you just moved.

Now your Outlook will be able to open, however you will notice that you now have a send/receive error!
To fix this you need to close Outlook and type “Control Panel” in the search pane at the bottom left of your screen to access your Outlook email setting via the “User Accounts” link in Control Panel.
Click the Mail (Microsoft Outlook) link at the bottom of the list.
Click the Email Accounts button.
Once in your email settings, click the email account that is not working. If there is nothing showing under the “delivers new messages to the following location” then click the “Change Folder” on the bottom left corner and click the “inbox”. Click ok and reopen Outlook. If you still get the same error, then repeat the procedure but this time add a new folder under the “inbox” such as Test. Click the inbox to highlight it and type Test into the Name field at the top. Click ok to close this Window. This action will add a folder called Test into the email folder tree. New mails we be downloaded to this test folder. Don’t worry as this is only temporary.
Click this test folder in the list to highlight it and then click ok to close the email account settings window.
Now open Outlook and your emails will start downloading to this new test folder as normal.

Lastly, in Outlook, move any emails that have gone to this test folder back to your inbox and then close Outlook. Again, go back into the email account settings that you opened before and change the download email location back to the “inbox” by clicking on it. Click ok to exit this window.

Finally open Outlook and it should be working correctly.
You can’t delete the Test folder from the email accounts window. You must do it from within Outlook itself. You can right click the Test folder and select to “Delete Folder”
One thing that I noticed, is that any Outlook email forwarding rules that you have previously set up, have been removed so you’ll need to create the rules again to direct the mail into the appropriate folders.
Please note that you must manually back up your .PST file every so often as it will not be protected.