According to mmafighting.com:

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.

Gegard "The Dreamcatcher" Mousasi (28-3-1)
Loss Muhammed Lawal - Decision (Unanimous) Strikeforce
Win Gary Goodridge - TKO (Punches) K-1
Win Rameau Thierry Sokoudjou - TKO (Punches) Strikeforce
Win Renato Sobral - TKO (Punches) Strikeforce
Win Mark Hunt - Submission (Straight Armbar) Dream 9

Jake O'Brien (13-3)
Win Toni Valtonen - Decision (Unanimous) FF 27
Win Dave Hess - Submission (Kimura) MMA Big Show
Loss Jon Jones - Submission (Guillotine Choke) UFC 100
Win Christian Wellisch - Decision (Split) UFC 94
Loss Cain Velasquez - TKO (Punches) UFC FN 14

Rumored card so far:
- Shinya Aoki vs. Tatsuya Kawajiri
- Gesias "JZ" Cavalcante vs. Katsunori Kikuno
- Alistair Overeem vs. TBA
Light-heavyweight grand prix opening round
- Melvin Manhoef vs. Tatsuya Mizuno
- Gegard Mousasi vs. Jake O'Brien

DREAM.15 coverage