Actually HO is 1/87 scale.
Now there is also OO which is 1/76 scale.
But don't get too caught up on true scale issues, do go with what looks good and works with your collection. Depending on the manufacturer BOTH model rail scale/guage kits can work wonders on a 20mm gaming table. And OO rolling stock is very nice indeed if you want to have some track and a train on the table.
I've used the same scale rail buildings with 20mm and 15mm minis with no complaints, so have really come to appreciate the versatility of these old hobby warhorses.
By contrast, Italeri make some attractive true 1/72 scale buildings that are simply too damn big to collect and play with, might as well be playing in 28mm at that point.