Both sides have verbally agreed to the fight, and barring any setbacks, contracts are expected to be signed shortly.
The fight is expected to be a part of the upcoming DREAM light heavyweight tournament, which will also feature an opening round bout between Melvin Manhoef and Tatsuya Mizuno.
DREAM.15 takes place July 10 from the Saitama Super Arena in Saitama, Japan, and will be headlined by Shinya Aoki vs. Tatsuya Kawajiri for the DREAM lightweight title.
Mousasi is coming off a Strikeforce light heavyweight title loss to Muhammed Lawal in April. The loss marked his first since August 2006, a stretch which saw him win 15 straight fights. His current MMA record is 28-3-1.
O'Brien has won his last two fights in a row since being released by the UFC following his UFC 100 loss to Jon Jones last July. The 13-3 veteran's most notable win is a UFN 8 win over Heath Herring in January 2007.
Gesias "JZ" Cavalcante vs. Katsunori Kikuno has also been officially announced for the card.