"Airwire in particular is aimed at G-gauge so it should have ample current capacity for O gauge."
Well...not exactly. O gauge is admittedly smaller than G gauge, but the amperage use is not proportionate to the size. Most modern (last 30 years) G gauge trains use very efficient can motors or equivalent. These use very modest amounts of current. Traditional O gauge (Lionel and clones) with series-wound motors use quite a bit of current when starting.