HO is actually half of British O-scale, which is 1 to 43.5, hence 1 to 87 for HO. Elsewhere in Europe, O-scale is 1 to 45 while here in the U.S. it is 1 to 48.
Now if you're not completely confused, just place your HO plan drawings on a copy machine and enlarge them to 181% and you'll have O-scale plans for 1 to 48.