If you are playing Soul Silver, Ho-Oh is also obtainable at the Bell Tower, but you need to obtain the Rainbow Wing from a person in Pewter City. Once you have it, you can go to the top of the Bell Tower and Ho-Oh will be there, this time at Level 70.
For the Clear Bell, you must get the Masterball from Prof. Elm, and head to Ecruteak City and battle the Kimono Girls. Defeating them gets you the required item. After that, you can catch Ho-oh at level 70.