I know this is a Judo throw(not BJJ) but I love this throw I must learn it!!
When the attacker punches, block and step through with a deep step — you need to get your advancing foot almost level with their rear foot. Turn through 180° while gripping under their rear armpit or by the belt if they're tall (and you're not). Using this hand, draw his balance forward, but not so he's bent over. When he's on the verge of falling, bring your leg up and backwards into his stomach, while continuing to draw him forwards — he should wheel about your leg.
As Sensei John Hamer will tell you, o-guruma is one of a continuing set of throws each of which is applicable depending upon how far you step through: koshi-guruma (if your feet end up between the attacker's feet), ashi-guruma (if you step through a little further), uchi-mata (if your feet end up either side of his rear foot) and o-guruma (if you pass his feet altogether). So even if you can't do the throw you originally planned because you've stepped through too far, there's always an alternative throw you can do instead.
Common mistakes:
* Not stepping far enough
* Not turning through enough
* Not getting the leg high enough
* Trying to execute the throw before you've taken the attacker's balance
* Losing your balance
* Making him bend in the middle
* Standing too far away from the attacker when trying to perform the throw
