This is only acceptable if you also say Android OS, Windows OS, Linux OS, and Mac OS (oh wait).
As for the domain name, we have been trying to get our hands on haiku.org for years, but the people who own it want something like $5000 to let it go. This is the only reason we are still using haiku-os.org.
The name of Haiku is just Haiku. Not Haiku OS. Where confusion with japanese poems is possible, you can say "Haiku, the Operating System", or "the Haiku OS". Like when confusion between windows (on a wall) and Windows (the operating system) is possible. But, that is not part of the name.