Ok, Quick answer. The natures and IVs are random, so use a Pokemon with the ability synchronize and keep resering for perfect IVs and natures
can influence its nature. Have a Pokemon with the synchronize ability
and the nature you want HO-OH to be. Encounter Ho-oh with the
synchronized Pokemon in front and there will be a 50% change it will
have the nature the synchronized Pokemon has. If the IV's aren't right
for you keep reseting and battling it until Ho-oh is to your liking.
Source^ from gamefax
Hope I helped:D