How do you pronounce oo in Japanese?

Answer 1

If you mean oo as you've seen it in a word (such as otoo-san) it's pronounced the same as "o" in Japanese. (O is pronounced like its letter, as the "oa" sound in "float")

The only difference between "oo" and "o" is that the double O is stretched out for a longer period of time, but not as though you're emphasizing it. It takes a while to get used to. :)

People have started to write double letters with a line accent over the stretched out vowel in the syllable, to make it less confusing. If you see two of any other vowel, it's the same, or with an accent.

Answer 2

In Japanese, "oo" is pronounced as a long "o" sound. It is similar to the English word "door." The sound is held longer than a regular "o" sound.