Sound and Syllables
The syllables in Japanese consist of a vowel A「あ」I 「い」U 「う」E 「え」O 「お」and a set of either a consonant or a consonant + Y and a vowel. There are exceptions, which are the small tsu 「きって - Kitte - Stamp」and the double vowel 「ああ(Ah) いい(Ee) うう(Ooh) ええ「えい」(Eh) おお「おう」(Oh)」 in the case of double e and double o it can be written ei「えい」 and ou 「おう」 respectively, however this does not change the way the word is said, it just extends the sound of the primary vowel. In the case of ei「えい」 it is still eh, and with ou「おう」 it is still ooh, just the sound of the primary vowels are extended, which of course is the case with all other extended vowels, though it is just the double e or double o that can use the i or u respectively along with a second e or o.
In Japanese, each syllable is pronounced with the same length unless the double consonants or vowels are present, and in the case of the double vowels it is just an extending of the sound of the vowel, but all of the characters in the word should still be pronounced equally.
「no double consonants or vowels」
it should however be pronounced in such a way that each sound naturally flows into the next character in the word
In the case of double vowels, it is important to realise that the extension of the sound does NOT change the sound, instead it gives the same sound as a single vowel word it is just that the sound is extended rather than altered like similar situations in English. Also, when a double vowel is written using roman letters 「called Romaji」the second vowel is removed and the line above the primary vowel and thus the sound to pronounce, denotes the extended length of annunciation that is required in that word, and if you see more than one, than each vowel which has it above requires that the indicated vowels' annunciation be extended as well.
In this case both vowels have the extension indicator above them so the "Oh" sound will need to be extended both times, which is different than Koshō 「こしょう・Pepper」which is pronounced Koh-Shooh, it is pronounced kooh-shooh, and remember, the extension does not change the sound of the vowel「which means there is no ooh sound involved with double O it just extends the Oh sound already present」, nor does it remove the need to enunciate each sound involved in the word.
ヾ( ~▽~)ﾂ Koyuuki ʿʿ˅⁽ˆ⁰ˆ˺ ⁾˺