I recommend you render the AO pass and do it is post production for more control.
1-Enable AO pass in render layers.
2-Open the node editor,enable use nodes and do the following.
- The RGB curve edit the contrast and the darkness of the AO which is helpful sometimes.
- AO is a Grayscale image so you have to multiply it to the render result.
In Cycles, the Ambient occlusion option in the World tab is technically an additional homogeneous direct lighting (almost as adding an additional solid white world background lighting). However the Ambient occlusion in Blender Internal can be set as additional shading (acts like multiply blending in image editor programs or in the compositor). The AO pass in Cycles is basically the shadow data only, which you can use the get the Blender Internal behaviour back, or use in other ways.