(All of this is very hard to describe using written words only, and many
people even speak differently depending on their dialect.)

Listen to the sounds on the Klingon CD. When Okrand speaks a syllable
like {bom} it rhymes more "bomb" than it does with "bo-u-m"

As an example of dialectical differences, no American English accent rhymes bomb with Klingon bom. Bomb has the same vowel as father.

I think the solution here may be that it's not that every Klingon o is a diphthong, just the syllable-final ones. The vowel in gho is a diphthong, identical to ghow; but the vowel in ghop is not a diphthong, and it doesn't sound like ghowp. Likewise with u: ghu sounds like ghuw; ghup does not sound like ghuwp.

This would allow what Okrand told you to be correct, with the exception of the syllable-final vowels described in the dictionary, an exception he wasn't addressing with you.

-- 
SuStel
http://trimboli.name