In Chapter 978 The Ninja-Pirate-Mink-Samurai Alliance has infiltrated Onigashima. In this panel we see the Thousand Sunny, which is approximately around 23 meters in height, meaning that the big sword seems to who knows how many times bigger than Oars who is around 80 meters, so it’s only logical that this sword belong to whoever that skull was in the past.
Here is another panel of Onigashima where we can see how insanely huge it is.
Here is my theory. Oars was an evil giant called the Continent-Puller who was born over 500 years prior to the current storyline. Legend has it that he amassed countries and islands under his control and his band of villains.
Onigashima meaning “den of demons” was the birthplace of Onis – the Oars ancestors – so one of Oars ancestors, most likely the skull of Onigashima created Wano Kuni by pulling multiple islands of the Grand Line together around his birthplace of Onigashima which would explain how those island all have different climactic changes.
The skull is most likely one of the first of his kind and the biggest one, most likely being born long before the Void Century and most likely died during Void Century, since I believe it will be explained then how Onigashima was created and what Wano Kuni is.
It could be that after each generation, the Oni descendants became smaller and smaller.
Or maybe that giant skull, ancestor of Oni giants, was the first user of Sanjuan Wolf’s devil fruit that makes him much bigger than normal giants
So this guy was 10-50-100x bigger than Oars, it would totally make sense that he was the one who originally had the title of “Continent-Puller”.
*Theory by RoronoaLuffyZoro