If you build a new fs and move your Oracle Home to the new fs, I think you need to run root.sh before Oracle will work. Root.sh is created when you first installed Oracle. If you don't have that file, you might have to reinstall Oracle.
Here is my guess, as I do not use HP-UX. File permissions are set by the user's ID as assigned by the fs, and not the user's name. When you rebuilt the fs, the fs assigned new ID numbers to all users. Therefore, the files are not owned by the new users. It might say the owner is "oracle" with ID 7, or some value, but the hidden number (like keycode) is different.