Pronunciation rules for letter combinations composed of vowel o

The combination of the vowel letter o and other letters can form seven letter combinations, namely oa, oi, oy, oo, ou, ow and r syllable letter combinations or, oor, oar, our and ower. We divide them into five groups for analysis: oa is the first group; oi and oy are the second group; oo is the third group; ou and ow are the fourth group; r syllable letters or, oor, oar, our and ower for the fifth group.