One Piece Odyssey is an original story set in the One Piece universe, but you will visit some of the story arcs from the manga/anime as part of memories. This page will detail which story arcs are present in One Piece Odyssey, along with a short synopsis of the events (without too many spoilers, for those new to the series).
Alabasta Story Arc
The first major story arc present in the game is Alabasta, a desert kingdom found in the first half of the Grand Line. Prior to the events in the actual kingdom, the Straw Hat Pirates met some members of the Baroque Works, a criminal syndicate that is attempting to take control of Alabasta. This arc is important, as it is Luffy’s first interaction with one of the Seven Warlords of the Sea, a group of World Goverment-sanctioned pirates. Baroque Works is composed of a very large number of people, with the top echelon only being known by a codename, a number. The group is led by Mr. 0, with the numbers going up to 10 (male members), and each number is accompanied by female agent, named after days of the week and holidays.
Water Seven Story Arc
The second major story arc present in One Piece Odyssey takes place at Water Seven. Actually, this is split into two arcs, with Water Seven taking place first, which will transition to Enies Lobby. Water Seven is an important arc, as the Going Merry (the Straw Hats ship) was breaking down and they were looking to get it fixed. Water Seven is a city that is famous for their shipwrights, hence why the Straw Hats visit it. This eventually leads to a new ship being constructed for the crew, the Thousand Sunny, as well as the backstory on Nico Robin.
Nico Robin, someone who accompanied and forced herself onto the Straw Hat crew, gets taken by an undercover government agency and brought to Enies Lobby. The Straw Hats go here to confront the Marines and World Government, with their plan to rescue Nico Robin. This is where Luffy debuts his Gear Second ability, which becomes one of his staples moving forward.
Marineford Story Arc
The third major story arc in One Piece Odyssey is Marineford, which is home to the Marine Headquarters. Prior to actually going there, Luffy’s brother, Ace, is taken into custody by the Marines and imprisoned in Impel Down. Luffy, with the help of another of the Seven Warlords, Boa Hancock, breaks into Impel Down to try and rescue Ace. However, by the time he reaches the area where Ace was being held, he had already been moved to the execution site. Inside Impel Down, Luffy befriends some of the prisoners, including Ivankov, a member of the Revolutionary Army.
Luffy and the prisoners eventually break out of Impel Down and make their way to Marineford, where the Whitebeard Pirates (Ace is a member of this crew) are already doing battle with the Marines, who have the Seven Warlords on their side. Things intensify once Luffy and the prisoners arrive, with a bunch of information revealed to the participants about Luffy and Ace. The war eventually ends, not without some losses, though.
Dressrosa Story Arc
The fourth major story arc that is in One Piece Odyssey is called Dressrosa. This is the second arc after the timeskip, which follows the Marineford arc. They go to Dressrosa after visiting an island called Punk Hazard, where they found out about horrible experiments being conducted. This eventually leads to the identity of Joker, a leader of the underworld. His identity is revealed as Donquixote Doflamingo, the king of Dressrosa. The Straw Hats make an alliance with the Heart Pirates’ leader, Trafalgar Law. The overall goal of this alliance is to take down one of the Four Emperors, Kaido.
After finishing the events on Punk Hazard, they travel to Dressrosa next to help take down Joker’s business. Joker’s business is producing artifical Devil Fruits, which has connections to a bunch of bigger pirate groups, like one of the Four Emperors, Kaido. Of course, Dressrosa has a deep history, with Doflamingo taking over about 10 years prior to your arrival. Without delving too deep, the Straw Hats eventually take down Doflamingo and stop his underground business.