One issue that we come across every now and again is where users/admins will move their PST files in to OneDrive, however what they don’t realise is that every time they receive a new email this creates a new version of the PST file and in time fills up the whole OneDrive.
If you also use retention policy’s this causes more issues as as per its job it will stop you deleting the files. In order to fix this you will need to first remove the OneDrive from your retention policy and leave this for 24 hours (as per Microsoft’s recommendation removal of the policy takes up to 24 hours). After this you will need to grant yourself access using a OneDrive administrator account by clicking the ‘access files’ button on the users profile in Office 365: 
After this you will simply have to go to the users OneDrive and find the PST then click ‘delete’, this usually takes 10 minutes however after a while should then delete!
