The entire plot of One Piece is an entire flashback where Luffy and Nami tell their grandchildren about how they first met, all of their adventures, all of the friends they made, all of their experiences and the other people that they met.
Side note: Luffy and Nami will officially adopt Otama by the end of the series.
Side note: Luffy and Nami will officially adopt Otama by the end of the series.