The One Piece OVA is a special, 30 minute "episode" that aired before Toei had even purchased the rights to produce the series. Thus, the animation and the voice actors are very different from the One Piece we are used to. It was produced by Production IG for the Jump Super Anime Tour in 1998, and was released on VHS exclusively to Jump subscribers later in the year. However, when Toei gained the rights to all production and distribution related to the anime, what little chance of the OVA being re-released became none at all. It will almost certainly never be released on DVD, so if you'd like to see it, please download KF's high quality VHS-rip at www.kaizoku-fansubs.com .
The episode is entitled "Defeat the Pirate Ganzack!", and sort of takes place between the Buggy arc and the Kuro arc, except for the fact that the crew is only in possession of one boat, while in the manga they had two at this point. The story follows the adventure of Luffy, Nami, and Zoro, as they fight the pirate Ganzack (basically Kreig with metal crab arms instead of guns) in an attempt to free an enslaved island.
For a more detailed summary of the OVA, please click here.