As of Arnold 5, the opacity, ambient occlusion, and flat shaders are color shaders. They don't support opacity. You'll have to combine one of those shaders with a shader that does handle opacity (for example, use the color from ambient occlusion with the opacity from standard surface).
I connect ambient occlusion to standard surface with opactiy, the beauty pass is fine , but i want to write this to the AOVs, it return black
Oh god..thanks a lot for this. I almost went crazy searching for the opacity option, and i coulndt find it since it was removed.
I once needed an AO + Opacity for the leaves of a tree. To make it work I created an AiStandardSurface and plugged an AiAmbientOcclusion(OutColor) to its Emission Color (Emission Weight = 1), and removed any Base color and Specular.
After that I plugged the alpha of my leaf image to opacity R,G and B.
Then in visibility for the object I turned off everything except Primary Visibility,Cast Shadows, and Self Shadows. This worked for me.
For Alpha Ambient Occlusion