Oars was a previous user of Nika Devil Fruit.

Explanation:

1. Oars was introduced in chapter 456 – has the digits 5 6.
2. Luffy’s shadow was chosen for Oars.
3. The following might be just due to Luffy’s shadow being in Oars, but I wanted to mention these points:– When Oars initially used Gomu Gomu no Pistol, he says, “I though my arm would be stretchy”.– In Oars’ Vivre Card, it was mentioned that his favorite food is meat
4. Oars’ name is based on the Norse God Óðr. Óðr was known to often go on long journeys.
5. The frozen large Strawhat which Imu was looking at might be Oars’. Oars was found frozen in the Land of Ice. He was still being kept frozen by Moria (to preserve the body). Headcanon: Maybe Imu is choosing to keep the hat frozen to preserve it/ doesn’t want to unfreeze it because it might get ruined.
6. Little Oars Jr was shown to wear a Kasa hat (made by Ace). There were a good amount of panels dedicated to emphasize on the Kasa hat.
7. Oars has an epithet “Majin” (“Devil”). Headcanon: It was given by the World Government. Celestial Dragons are shown to be gods, and people who oppose them are termed as devils.
8. Oars is also known as “Oars the Continent-Puller”. Headcanon: He brought together land fragments atop of Wano. Reason – Wano was on sea level at some point in the past. Walls were erected around it probably to protect it from the initial flooding (some time during the Void Century). Due to the walls, the land of Wano was slowly starting to submerge due to rain. When Oars was around (around 700-500 years ago), he solved this problem by creating livable land higher up – with the help of Nika fruit. Side note: It was mentioned that Oars took over islands and created a nation of villains (probably a narrative spread by the world government). He took other islands and merged them together to create the new Wano. The nation of villains in this case would be Wano.
9. Headcanon level 100: One of Oars’ ancestors might have been “Joy Boy”. Oars’ is classified as “Ancient Giant” maybe to hint that his people are from the Ancient Kingdom. In Chapter 1114 it was revealed that Joy Boy was from an advanced kingdom. During the Void Century, his side lost – causing all the advance technology to be lost as well. This might be why Oars was very primitive (and was from a different sea – North Blue. His people might have migrated after losing). The Ancient Kingdom might be Elbaf – where there were much larger giants (ancient giants) before. The myth of Nika is very prevalent in Elbaf – due to which the Nika fruit was created (from the dreams of the people) and eaten by one of the Giants. The first person who ate the Nika fruit would have adopted the personality of Nika and went out on a journey liberating people – causing the person who ate the fruit to be known as the first pirate.