* Ho-Oh's location is tied to the legendary dogs. In Crystal, Ho-Oh appears in the Ecruteak City Tower only after you've caught all three legendary dogs (Raikou, Entei, and Suicune) and triggered a specific event. This event requires you to have all three dogs in your party and then talk to the elder in the tower.
* Specific events trigger Ho-Oh's appearance. The encounter with Ho-Oh is heavily scripted and relies on these events happening in a particular order. You can't skip the dog encounters and directly access Ho-Oh.
To catch Ho-Oh in Crystal:
1. Capture Raikou, Entei, and Suicune. This involves tracking them down in different locations across Johto.
2. Return to Ecruteak City. After catching all three dogs, head back to the city.
3. Talk to the elder in the tower. You'll find the elder near the top of the tower.
4. Ho-Oh will appear. The encounter with Ho-Oh will trigger, allowing you to catch it.
Important Note: If you want to catch all the legendary Pokémon in Crystal, you'll need to play through the game twice. This is because you can only catch either the legendary dogs or Ho-Oh in a single playthrough.