It first aired on July 18, 2010. In contrast with all of the series' openings up until now, "One day" is more somber in tune. This could be attributed to the events that occurred during the Marineford Arc.
The One day opening is mainly divided into 4 parts:
The first verse displays each Straw Hat alone at one of the crew's tasks before the traditional gathering.
The first and second choruses display group shots of the different Marineford Arc characters. Save for Rayleigh and Shanks, all of them were shown in a montage with a close-up image of their faces displayed from the background from each of their respective entry.
It displays the Straw Hats doing various things: Zoro cleaning one of his swords, Usopp sitting on the railing of the ship fishing, Nami somberly stirring her tea and looking at her Log Pose, Sanji inspecting some wine in the kitchen, Chopper working on some medicine, Robin reading a book, Franky working on a cannon, and Brook playing his violin at the rear of the ship. A group shot of the whole crew is then shown.
Whitebeard's crew shots: First a display of some of the major Whitebeard Pirates (with Whitebeard in the center) and some of their New World allies. Everyone but Whitebeard fades out, and Whitebeard then uses his quake powers to crack the air. Then comes shots of Marco as a phoenix, Jozu covered in diamond, Vista with his swordplay and rose petals, and Oars Jr. roaring at the screen.
At the song opening:
It begins with a shot of the Straw Hat Jolly Roger as it blows in the wind. Then it goes to Luffy as he rests on the roof of the crow's nest. Then it goes to a shot of the back of the Thousand Sunny as it sails on before cutting to Luffy and Ace facing opposite directions as the title comes up. We then see Luffy walk through some rubble before a few shots of Ace.
At the end of the first verse:
We then cut back to Luffy who continues walking through the debris as he perks up because he spotted something and smiles before he starts to run.
At the beginning of the second verse:
Luffy still runs through the debris smiling.
The last few seconds of the second verse:
Luffy continues to run through the debris from behind.
Second chorus ends:
We finally see who Luffy was running to: Ace, who has his back turned to him. He turns around and, though we don't completely see his face, he smiles at him. Luffy does the same before we get a wide shot of the two and the One Piece logo ending the opening.
The band The ROOTLESS had always dreamed of some day doing a One Piece opening. They wrote the lyrics through trial and error, with no less than twenty rewrites, arriving at the message of "protecting the things that are important to me".
The group shot of the whole crew at the end of the first verse is inspired by the cover from Volume 52 of the manga.
The One Piece logo is dramatically modified for the first time in concurrence to Volume 57's cover, in which most of the first half of the Whitebeard War takes place.