A Microsoft file hosting and synchronization service.
I'm not sure if this will help any of you out but I was having the same issues with Dropbox and One Drive. I thought both were taking up most of my HD space even though I had all my files set to online only. I have a brand new iMac that has 256 HD and storage was saying that 116gb were being taken up by "other". I am a graphic designer and use photoshop every day. I then quit photoshop and watched that 116 go all the way down to 27gb. photoshop was using my HD as a scratch disk. I am now going to get a TB external and set that as the scratch disk for PS. voila!
not sure if any of you are also using PS but this took care of the problem for me. there is still 23gb being taken up by "other" but I can live with that.