Strike Witches OVA was the very first animated iteration of Strike Witches. A promotional DVD with prototype animation was sold by Toranoana at Comiket 71 on December 29th 2006. This DVD came with a PVC figure of Yoshika Miyafuji and an illustration book of Humikane Shimada's works titled STRIKES, as part of the "Mechanized Maiden 2²+3D Box". The packaged set was later sold at Toranoana stores, starting on January 1st 2007.
The OVA abbreviation stands for "Original Video Animation". It is not to be confused with Operation Victory Arrow, which is a short series of three animated episodes; often given the OVA abbreviation as well.
The first 8 minutes of the DVD contains the original OVA, with the remaining 16 minutes footage being a more detailed explanation about the show; such as the base, Striker Units, weapons, and some of the supportive characters.
However, the setting design and visuals of certain characters, along with the general art style, was slightly changed in the main anime. In addition, the scenes depicted within the first 8 minutes of this OVA should not be considered entirely canon either, as this was only a pilot episode and not related to the later events of Season 1 and onward.