On18 (18 inch gauge in O for very small industrial stuff) is done using N scale trucks and locomotives to run on N gauge track.  N gauge track is not very scale for this so you need to remove a lot of ties or hide them in balist.

On20 ( 20 inch gauge in 0) is done with HOn3 track and trucks.  Because HOn3 track is narrow gauge to begin with it works out a little better for scale.  I have some MDC HOn3 2-8-0 kits I plan to use for this by adding O scale cabs and tenders.

If you use S scale track for O scale narrow gauge it comes out to 42 inches or On42.