While the 'o' sounds may be long in Osaka and Tokyo, I would be hesitant to pronounce them as two separate letters as appears to be suggested

No, I wasn't suggesting that: to clarify, Tookyoo is two syllables, Oosaka is three, and Aomori and Yokohama are four; but in Japanese the mora or beat is more important than the syllable. The first of a double consonant is also a beat: so Ni-p-po-n is four beats, same as Yokohama. (And it's three syllables because the final -n is a separately pronuncible syllable.)