i do my AO slightly differently...


if you want an AO pass, just select everything, go to render next to the layer bit in the bottom right side of the screen. click on that and you are in the render layer bit.
make a new layer assigning everything to it. (its the button with the blue ball on it)
right click go to presets> occlusion

once you have done that just find the occlusion node (click on tte surface shader in the hypershade and then click on the out arrow on the out colour. change the samples to whatever you want, but remember to change the max distance to something smart like 5 or whatever -depends on the scene-

render.

occlusion done.