If the settings are updated in real time, the "OK" button is superfluous: the change is already made. Likewise, you can see the results instantly, and there is no need for an "Apply" button to preview the change.
The "Apply" button is perhaps best seen as a performance compromise on early, underpowered machines: the update may take a while, so it's more efficient to group multiple changes together, and to leave that update (which will likely freeze the application for the duration of the update) at the will of the user, rather than doing it arbitrarily.
(Some applications take the best of both worlds: GIMP for example begins updating the image preview shortly after entering a change, but interrupts the update if another change is made, and so on. This thrashes the CPU, but cores are plentiful these days, so it's no problem for responsiveness of the OS or the application.)
A "Cancel" button, to revert the settings back to the state when the dialog was first opened, would still be handy. This is excusable on the basis that, if you're seeing your results in real time, you'll know precisely which change had which effect, and it's reasonable to expect the user to remember the previous value of one item at a time.
Tim