Others have aptly replied to your language question but I sense another
question in your "assuming openbsd 5.4 amd64 box here". Namely: "how to
I install an Ada compiler and assorted development tools on OpenBSD
5.4"?
Well, I'm afraid you won't like the answer, which is, basically: you
don't. John Marino had a word about that at FOSDEM, where he said he'd
done all the ground work to get GNAT ported to OpenBSD, but the OpenBSD
people were not interested enough to spend the time and commit his
contribution into the official ports tree.
In contrast, some other distributions have excellent, built-in support
for Ada: Debian, FreeBSD and Dragonlace are currently the best. (Debian
includes Debian GNU/kFreeBSD, of course, if you're allergic to the Linux
kernel).
More details in the _Ada Programming_ wikibook, which, by the way, could
also be a way for you to learn the language:
https://en.wikibooks.org/wiki/Ada_Programming/Installing
--
Ludovic Brenta.