So, Oda’s showing us the graves of four characters we’ve met already, and of one character whose death these other four mourn. Now, the way Law is speaking about them, it seems pretty clear that they died and are now ghosts. But if Kinemon, Momonosuke, Kanjuuro, and Raizou all became ghosts after they died, how did lord Oden not end up a ghost?
I have a theory about this. Oden was probably actually executed by boiling. His retainers, knowing they’d be the next to go, probably faked their own deaths and the death of their lord’s son, so they could escape. After that, they worked in secret for years, trying to set everything in motion to overthrow the Beast Pirates and place Momonosuke on his father’s throne. So these stones actually stand over empty graves.