I agree with Rich, but would go further. 18-gauge wire has to place on an O-gauge layout track feeder circuits.
Mannyrock. For toggle-switching blocks, run the power from transformer to a control panel via 14-gauge. On the panel place togglew switches for each block. Run 14 gauge from toggle switches to each block. For common, run a 12-gauge buss all around the layout, back to (and connected to) its point of origin.
Breaking up the ENTIRE layout into blocks provided a better degree of control when running conventional. Years after I built my current layout, I plopped in DCS and made few wiring changes.