According to wikipedia O guruma is classified as a foot throwing technique. However my understanding was that the arms do the main work of breaking the balance and the leg acts as a block/pivot. Is my understanding of the technique incorrect?
Throws in Judo are typically classified/named in the original Japanese by the fulcrum or pivot center, not the point(s) of force origin. Sometimes the name of the move also involves a metaphorical description of any significant body motions imparted on the uke to help illustrate the overall look or feel of the move from the stand point of the person who is experiencing the throw.
So in this case, the move is called Ō guruma, approximately meaning large or big wheel in English, because the uke is thrown over the leg such that they spin in a large vertical wheel. See this video for a good example.
It is classified as Ashi-waza, meaning both leg and/or foot technique, because the leg is the point about which the uke rotates.
To reiterate what Dave said earlier, pretty much all throws involve the contribution of the entire body when it comes to the application of force (whether to off-balance, re-position, or provide the tipping force), but there is only one pivot point and this is what is used to classify throws.
The arms do the work of unbalancing in most, if not all throws. That includes ashiwaza such as okuriashibarai, osotogari, and quite obviously sasaetsurikomiashi. These throws would be unworkable without breaking the balance through the arms. That doesn't make them arm/hand throws, because the telltale element of leg throws is that the leg is the pivot or dominant factor in the throw.
Contrast this with hand throws, where the arms don't just unbalance the opponent, they are the spot where the action occurs. For instance: tai-otoshi, where the hip and leg do not make contact, seoinage, where the shoulder or back is the pivot, or others like kataguruma, sukuinage, or morotegari, where the arms are the point of action .
Just like arm throws involve significant leg work (e.g. kataguruma) or hip action (e.g. sukuinage or teguruma), leg throws almost always involve a great degree of arm work. All throws should optimally be involve a coordinated whole-body effort. Oguruma is a leg throw because the characteristic element of the throw--what makes the movement worth studying as distinct from other throws--is where and how the leg turns the opponent after a state of broken balance has been achieved.