Op Thorb (オプ・ソーブ) is the third and sixth boss from Aleste 2.
At first, this boss will act much like the Ground Bases from Zanac and the original Aleste, with the four ground turrets spreading bullets. Once the turrets are taken down, the core will open, releasing a multi-segmented snake-like robot which will move around the screen while shooting at the player. In order to destroy it, you first have to shoot it's segments, which will change color until they become white, with the head of the robot becoming vulnerable once all segments are white.
For the rematch in the sixth level, instead of the domed structure from stage 3 you fight a giant mutant stuck to the ground. There are various openings in the ground, from which the snake robot will constantly exit and re-enter, shooting the player in the same manner as before while the monster's mouth also shoot bullets.