How to Get Ho-Oh

1.  Have the 3 Legendary Beasts in your party (Raikou, Suicune, Entei) then fly to Chemsa Town and talk to the woman next to the man near the squirtle water fountain.

2.  You should have the Rainbow Wing after talking to the woman. Now that you have the Rainbow Wing, fly to Cragonos Cliffs and walk to the edge past the flowers where Shaymin spawns and a cutscene should start with the Rainbow Wing and Ho-Oh should pop up.