Serbian 101

oa

the 'oa' in "boa"

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ao

the 'a' in "another" and 'o' in "open"

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eo

the 'eo' in "video" ('e' as in 'there')

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io

the 'io' in "radio"

Note that the diphthong ia is usually written as ija; eg, policija (police) is pronounced exactly as Italian polizia; name Lydia is written as Lidija; Serbia is, as you have well guessed, spelled Srbija.