There's actually a few different ways of doing it, each with their own supporters. The one used here "ou" can be seen as incorrect, as it is actually the combination of two distinct sounds ("o" and "u"), rather than the truly long "o" sound, giving a pronunciation of "Ai Ki Doh Ooh", instead of "Ai Ki Doh Oh", which is more accurate. That I have seen written as a double "o", leading to "Aikidoo", but due to a Western reading leaving that as "Aiki Doo", it's not entirely trustworthy either. The last, and probably most accurate, is to simply use an accent over the "o", extending the sound of that letter/syllable, with a writing of "Aikidó"
Of course, that's if we're going to get technical...