Hi
The most important question in several ways is, how we can apply one mat in multiple nodes in Houdini?
I have at least 15 nodes that should be rendered by Arnold !!!
how can I apply Ao or ambient occlusion shader on them?!

First mutli-select the obj nodes you want to assign. Then, in the Render tab of the properties pane you should see a Material parameter. Clicking the far right icon will pop up a panel that lets you browse to whatever shader you want to assign in the mat context.