This is just all speculation, but after re-reading the chapter, knowing that O’Tama came on an enemy ship and didn’t elaborate further, it has me wondering if this could be Shanks’s ship.
The last time Shanks was seen was at Mary Geoise, talking to the Gorosei about a certain pirate in chapter 907, and before in chapter 903 talking about meeting Luffy soon after Luffy had been labelled a fifth Emperor.
Two weeks have passed in world, which could give enough time for the Red Hair Pirates to come to Wano. Like Kid, Shanks may have deduced that Luffy was targeting Kaido after the events of Punk Hazard and Dressrosa and was in Wano to face off against the strongest creature.
One reason I can think of the Red Hair Pirates coming to Wano would be the events that occurred during Bartolomeo’s section of the Straw Hat Grand Fleet cover story, where he landed on one of Shanks’ islands and proceeded to burn the Red Hair Pirates’ flag and sell Straw Hats merch to the residents. But I think this is a rather weak reason at the moment for Shanks to be present, and be another antagonist in the Fire Festival.
The stronger argument of Shanks being in Wano is Oden. I have always found it odd, after seeing Oden’s flashback, that Nekomamushi went to recruit Marco for the fight against Kaido, but didn’t think to do the same with Shanks. Nekomamushi and Inuarashi were both on Roger’s ship, and knew Shanks. They would surely know that he is one of the four Emperors, and would be a huge asset in their fight against Kaido (and Big Mom). Obviously, the timing of when Nekomamushi was seen with Marco was when Shanks was at Mary Geoise, but there could have still been enough time to meet/contact Shanks. It could also be possible that Marco may have contacted Shanks as they are on friendly terms, especially after Shanks helped with the burial of Whitebeard and Ace.
Shanks and Oden were close, as seen during the flashback. Shanks could have found out what happened to Oden from Marco/Nekomamushi (if Marco contacted Shanks, that is), and what was planned in the near future. Unless something happened between the minks and Shanks that we are not aware off, and they chose to not contact him, though not sure with Marco if he chose not to.
My theory after reading this chapter is that Shanks could be shown in Onigashima during Chapter 1000 (maybe even on the roof of the dome, and he and Luffy finally meet, since Chapter 1). He and his crew are benign enough to have O’Tama aboard their ship to help her from Wano to Onigashima (unlike Luffy who wanted to join Shanks’s crew), though she could still view the Red Hair Pirates as an enemy. This could tie in with the promise by Luffy to Shanks, meeting up when Luffy becomes a great pirate, and Shanks now views Luffy to be so (as it has been nearly 100 chapters since Shanks stated this).
Shanks, like Kaido and Big Mom, is part of the older age of piracy (though he came to prominance during the great age of piracy, whilst Kaido and Big Mum did around Roger’s time or after). This ties in with the Yonko saga and the Worst Generation defeating them.