Sentai Filmworks is an anime licensing studio under Section 23 Films, specializing in japanese anime shows and films. It is considered a successor to the now defunct ADV Films. It currently holds the rights to Big O, though they've yet to give the show a home media rerelease as of July 2015.
When distribution company Bandai Entertainment ceased DVD production entirely, Big O and many other shows licensed by the company for stateside distribution went into licensing limbo. Sunrise eventually announced that they would pitch their shows to other distribution companies. The rights to Big O would go to Sentai Filmworks.