Is it possible to run part or all of a tournament off OGS, using in-person game results and ranks, I guess use just the organizational aspect of it?
No. I think you’re looking for something like this:
It would be possible for you to create accounts for each tournament participant. You could then log in to both accounts and then recreate the real life games. But there would be no way to modify the ranks of each account to represent the real life ranks of the participants.
It would essentially be a really complicated way to create a tournament tree structure for viewing tournament results, as well as digital copies of the matches themselves. It would be a lot of work. There have to be much better software solutions out there for this sort of endeavor.