One Piece is a lengthy pirate epic filled with glorious adventure, heartbreaking tales of the Straw Hats' journey across the seven seas, and, last but not least, loads of fan service. If you're unfamiliar with what exactly the term "fan service" means, you might be able to guess what it is just by the terminology itself.
Whenever a show like One Piece does something explicitly to get a rise out of the viewer, such as when Nami or Nico Robin are in the middle of a conversation and their boobs inexplicably jiggle, it's 100% fan service. Here, you'll find prime moments of obvious One Piece fan service made for your viewing pleasure. Now, you can fondly reminisce about all those times where the Luffy and Nami fan service in One Piece got so in-your-face you almost stopped caring whether or not they ever actually find the One Piece treasure. While the fan service in One Piece is usually quite blatant, that doesn't mean it's not fun.