Hi @alex,
Do you happen to use Time Machine for backup? I have seen cases where Time Machine’s local snapshots start taking up lots of space. Take a look at this post for more information: Drive space not recovered after unsync on Mac
What is reported when you run this command from a terminal window on your Mac?
df -h
I also found this post, which goes over several potential possibilities for this. It also notes that you may need to run Omnidisksweeper with elevated privileges (sudo) to make sure it can look at the entire disk: https://www.reddit.com/r/apple/comments/3q53dk/100_gb_of_other_on_macbook_pro/cwc4rxf/