if you go here https://outlook.office365.com/mail/options/accounts-category/storage it will show you the largest folders in your mailbox and gives you some preset options to clear 3, 6 or 12 months and older (or all) to free up some space, you can leave this processing in the background. Usually deleted & sent are the largest folders:
Note. During large deletion operations it can causing outlook crashing for those people with access to your mailbox, it's usually best to ask IT to temporarily remove their access until deletions are complete