Yes you are correct transportable tablespaces won't work either. It has to be the same OS. :( That feature is great if you have a prod and dev database on the same OS.

On compaq: exp system/bla bla file=full_db.dmp full=y
ftp: full_db.dmp to HP.

On HP: imp system/bla bla file=full_db.dmp ect ect.

As I'm sure you're aware there's other things you need to look out for like file size and such but that's the jist of it.