The wiring will either be run down the center underneath through pipe hangers like I did for the standard gauge, except smaller (see photo). Or, I may run the wiring under the standard gauge and pop up to the O gauge where needed.
I use the pipe / conduit holders as well. I usually leave one side unscrewed so I can run the wire and then slip it in. It's a good solution for hanging wire though regardless of 1 or 2 screws..