According to AT-X Shop, the episode 13 of "Sengoku BASARA
" will be released as an OVA. It's separately packaged in the volume 7 of the DVD series and will be released on January 6th, 2010.
The episode 13 is a side story. Maeda Toshiie and Matsu visit Chousokabe Motochika
, who governs Shikoku
. They tell Motochika unreasonable demands from Nobunaga. Motochika rejects them and urges Maeda to betray Nobunaga.