LPT: Freeing up space from offline files using Everything
Just sharing this tip as I just figured it out - I've never found the "Free Up Space" option in OneDrive to work particularly well, it often doesn't seem to do much of anything to actually remove the downloaded files from my computer. If you've had similar issues then this is a way to specifically target and undownload files that might be taking up room on your PC. Handy especially if you have a large amount of folders in Onedrive with downloaded files scattered all over the place.
If you're not familiar with Everything then I'd highly recommend it. It's a powerful windows search tool that gives instant results. For this particular trick you will need to have version 1.5 beta installed.
Run a search on your Onedrive folder and in the search bar type !attrib:o This will display all the files in your Onedrive that are downloaded to your PC. The "O" attribute is given in Everything to online-available files, so searching with the ! will list all files that do not have this attribute. From there you could select all, right click and choose Free Up Space which will put all these files back to online only. You could also sort by file size and get rid of the largest offenders, or however else you want to tackle it.
Hope this helps someone one day, or maybe there's an even easier way within Onedrive itself but I've never seen it personally.