The AO node doesn’t appear in 2.79b+ or in 2.8 for me. Is there a setting that needs to be enable to show the AO node?
it is not AO it is Ambient occlusion!
Are you doing SHIFT-A for Add then search, or are you doing SPACE or F3 to do a command Search? It shows up in the former but there are no commands with ambient in the name for the latter to find.
The size of your search box suggests this may be your problem.
Ah yes I was doing space for search instead of shift + a then search. It shows now