Both O’s in “zoo” are needed to create the “oo” sound in this word, but the same is not true of words like “zoology” and “zoologist.” Here each O has its own sound: oh followed by ah. The first two syllables rhyme with “boa.”

Then there is a whole class of technical words like “zooplankton” where both O’s are pronounced “oh,” though the second “oh” is pronounced so weakly it comes out more like “uh.” But if you need to speak such words, you probably know how to pronounce them already.