AThe studios Satelight (OVA 1 through 4), Madhouse (OVA 5 through 7), and GraphinicaxKelmadick (OVA 8 through 10) elaborately worked closely together with Kohta Hirano. As a result, they created the OVA (Original Video Animation) of Hellsing which in terms of the story stays very true to the original material. Each OVA is an almost one-to-one transfer of an entire volume of the manga. The animation is top-notch, just as the music. Thus, the OVAs, which include all the epic battles, are a very worthy transformation from Hirano's blood-soaked comics to the TV screen. In Japan, 9 OVAs were already released on DVD, while the tenth OVA is supposed to be released somewhen in 2012.
The OVAs from part 6 onwards were also released on Blu-Ray. In October 2010, a Japanese box-set was released which also includes the OVAs 1 through 5 in 1080p. What makes this box interesting are the completely revised "Recut" versions of part 1 through 4. They really worked hard and with a lot of sensitivity in order to make the quality of the animations of the first episodes just as good as the quality of the later ones. The color as well as the image brilliance was noticeably improved. Backgrounds, landscapes, eyes and glasses have much more detail, light, and reflection. Also, a few mistakes of the facial movements were undone (especially those of Alucard in OVA 1 and 3 but also those of Luke Valentine in OVA 2). All in all, the first four parts now have a completely different atmosphere to them and now look great in comparison to the more recent episodes.
Detailed comparisons between both versions with pictures can be found here.