I built an outdoor O layout a few years ago. 3 loops, Atlas track.
One piece of advice: I should have soldered the track joiners. When winter in CT is over, I have to work for a while to make all the connections work again. Problem is that the joiners get corroded where they meet the track sections. Had I soldered then joints when new, it would have been easy. Now I would have to take the track apart, clean the ends, add joiners and solder while clean. Joints won't solder once a layer of corrosion is on them.
Other than that, the trains run great outside! Mostly MTH DCS running...