The spelling of the word "ONOO" can be explained using the International Phonetic Alphabet (IPA) transcription. The first syllable "ON" is pronounced as /ɑn/ which is the same sound as in the word "on". The second syllable "OO" is pronounced as /u/ which is the same sound as in the word "food". The final syllable "O" is pronounced as /o/ which is the same sound as in the word "go". Therefore, the word is pronounced as /ɑn.u.o/.