I guess I'll just level myself out of the possibility of fighting O&S again in jolly co-op
Where do I leave 4 kings summoning signs, firelink?
The two main locations for that boss are in the little courtyard area immediately after descending the stairs from the elevator and right outside the boss door. Sometimes people also leave them on the small bridge leading to the first ghosts, but I've always thought that seemed needlessly out of the way.