Rurouni Kenshin OAV is divided into four episodes. The first two comprise the 'Kyoto' arc, the second two the 'Otsu' arc. Each episode is different in content--the first establishes Kenshin's character and background, the second covers the political aspects of the story, the third is about Kenshin and Tomoe together and the fourth contains the stunning climax and what comes after; the series continues well beyond where a lesser one would stop in order to provide a proper, conclusive and satisfying ending. The pace of the story constantly changes over the four episodes but it is so skillfully constructed that these changes in tempo never disrupt the narrative in the slightest. The story is fantastically engaging, if difficult for people unfamiliar with Japanese history to follow--I would not recommend this anime to non-hardcore fans simply on the grounds of it being too Japanese. It is driven by its characters more than by its action, although not in the usual sense. This story revolves around two characters--obviously Kenshin and Tomoe--but the way they are handled is completely unique.
As a result of this, and indeed as a result of its status as an OAV, the Rurouni Kenshin OAV (more widely known as Samurai X, although not here) is different from the Rurouni Kenshin TV series in almost every way imaginable. The most notable and significant difference is in the tone of the content. Because the OAV deals with Kenshin's time as a merciless assassin it would obviously be very violent; the oath against killing has to go. This series is dark where the TV series is funny and grim where the TV series is warming. Where the TV series is meant to entertain, the OAV is meant to enthrall. The nihilistic tone is set with the first words spoken by Hiko: "They are afflicted." Even the character designs are different--Kenshin has hair that is different in both style and even in colour (a darker brown than in the TV series), dresses in darker clothing, his eyes are more realistically proportioned and have a hard, deadly look to them. The rest of the animation is different: there is none of the semi-SD which was in the series--it was lost with the rest of the humour--and the animation is sharper and more detailed. To complement these differences, you can watch the Rurouni Kenshin OAV without having seen so much as a single episode of Kenshin--I myself had only seen one and it was totally unnecessary. Although the story of the OAV does spoil parts of the TV series' plot it isn't really important as the OAV is better than the TV series anyway.
The voice acting and translation is so important that it needs a paragraph to itself. In a lot of anime, dub or sub is a no-brainer in favour of the subs, but never has it been so much an issue as in the Rurouni Kenshin OAV. Both versions possess quality voice casts, although my preference lies firmly with the Japanese actors. Not all of the voices were chosen to copy their Japanese counterparts, most notably that of Kenshin himself. Mayo Suzukaze's voice is soft, calm and emotionless, even as Kenshin tells his mark of his oncoming death. The voice is so appropriate that it is almost impossible to remember that it belongs to a human instead of an anime character--it makes Kenshin seem alive. In the dub, Kenshin is voiced by J. Shannon Weaver who also voices Okita. His voice is pitched fairly lower that the Japanese voice and he has a slight, but noticeable American accent, unforgivable in period piece characters. He delivers his lines much more harshly than in the Japanese version, a recurring theme I'd noticed all throughout the dub. Regardless, he still does an excellent job providing Kenshin's English voice since he managed to get the tone right (although the less said about his voice for Okita the better). Both Junko Iwao and Rebecca Davis do excellent jobs on Tomoe, although the Japanese actress comes out on top again, this time due mainly to the "Japanese-ness" of the anime. Some of the dub voices--Katsura, Saitou, Hiko and Tatsumi--are absolutely spot on. On the other hand, some are, regretfully, rather sub-standard, especially Enishi, Katagai and the aforementioned Okita. It all evens out into a dub which should be quite acceptable to your ears. The fact that several lines in the sub are, for some unknown technical reason, inaudible also adds to the value of the dub. However, the most important differences between dub and sub aren't in the voices, but what they're saying.