According to the official website, two-volume OVA of Amagami SS
will be distributed to those who purchase all the volumes of the DVD/Blu-rays of the TV series. The first volume will carry three short episodes of Haruka, Kaoru and Sae. The second volume will contain Ai, Rihoko and Tsukasa’s episodes. The 24 episodes of the TV series will be released in 12 volumes of the DVD/Blu-rays. The final 13th volume, which will be released on April 20th 2011, may be an OVA as well.
Source: Amagami SS official website
, Amazon Japan