1. The National Pokedex: You can only encounter Ho-Oh after you've completed the Kanto region and obtained the National Pokedex.
2. The Rainbow Wing: You can only encounter Ho-Oh after you've obtained the Rainbow Wing, which is located at the top of Ecruteak City's Bell Tower. You'll get it after you've completed the "Entei, Raikou, and Suicune" event, which you'll be prompted to do after obtaining the National Pokedex.
Once you have both the National Pokedex and the Rainbow Wing, head to the Tin Tower in Ecruteak City. Ho-Oh will be waiting for you.