To perform this throw, you use your right leg to sweep both of your attacker’s legs away. Which is like o-soto-otoshi.
What’s the difference? In o-soto-otoshi, you knock the attacker backwards while sweeping his legs. In o-soto-guruma, you take the balance towards the side more, so that your leg isn’t behind both legs when it reaps them, but to the side of the attacker’s right leg (if he attacked you with a right-hand punch). The result is that the attacker’s right leg sweeps his left leg along and instead of wheeling backwards, the attacker wheels sideways.
Ok, there isn’t much difference if you are defending in a tight corner!