HO track typically is set up for 2" separation (center-to-center) for straight parallel tracks (wider spacing is needed on curves). Whether that is enough for your On30 equipment - it probably isn't - depends on the prototype you are following. 2" = 8ft in O scale, which ain't much. 2" = 14ft in HO scale, which is very close to prototype spacing used in yards in the 1950s.
If you look at the NMRA standards for track spacing (
NMRA - S-8 Track Centers), you will see that On3 calls for 2.75" on straight track. If you are running accurate models of 3ft gauge (on On30 track) K class 2-8-2s or large outside frame 2-8-0s, you will need every bit of the NMRA standard - and even might need some more than the standard on curves.
OTOH, if you are running strictly models based on 2ft prototypes or smaller 3ft prototypes, I have heard of 2.5" being used successfully.
Last point on clearances - all the prototype narrow gauge track I have looked at used considerably wider than required spacing in yards and towns. I have rarely seen tightly packed tracks like in a standard gauge yard - Alamosa may be the exception.
Also, a word to the wise - vertical clearances for On30 are going to have to be bigger than HO.
hope this helps
