This is a great thread topic. I was actually going to post this exact thread about 2 weeks back but thought it would be lacking in discussion.
But as many people pointed out already, Oonoki can transfer the ability to others, so advanced chakra manipulation on his part is not the key to his flight. It is clear, however, that his "super light weight rock technique" is a primary factor in the equation. Obviously it can apply to anything, not just rocks, but the most important thing to consider is what happens after he decreases their weight. This unknown propulsion is the key.
Now it does not seem to me that his dust release is whats propelling people. Rather, it seems that anyone, or anything, that is granted this 'super light weight status' has the ability to propel themselves. From all this, I can only conclude that the propellent is simply a person's concentration, or perhaps the flexing of their muscles (yes, kinda like superman). But I think Oonoki can decrease an object's weight to zero up to a certain limit and than once that task is done, the object or person in question can propel themselves at will.
And as I just eluded to, there do seem to be limits that even Oonoki has to abide by. When he lifted the island, it was commented that he was overdoing it and obviously he could not stop two of Madara's meteors at once so there has to be some limit to what overall weight he can take away from a given object. But all in all, I think this may be another key to the SO6P question and a great topic to bat back and forth.