Glocke

Ho-Oh is the Pheonix Pokémon and the star Pokémon of Pokémon Heart Gold. As such, the way you discover it in Heart Gold is
through obtaining the Gold Feather after beating Team Rocket and defeating the Kimono Girls. With this, when you go to the Tin Tower in Ecruteak City and climb to the top, you will encounter Ho-Oh at Level 45.
If you are playing Soul Silver, Ho-Oh is also obtainable at the Tin Tower, but you need to obtain the Gold Feather from a person in Pewter City. Once you have it, you can go to the top of the Tin Tower and Ho-Oh will be there, this time at Level 70.