Mr. Green is symbolic of the European presence in Nigeria, as he is the epitome of the "paternal colonizer," who has brought some good but mostly arrogance. He is very much the kind of Englishman who believes in the good of empires and thinks he can, as Obi points out, tell people how to live their lives.
If Mr. Green stands for Europe in Obi's struggle between tradition and European ways, then the UPU stands for the stubborn traditional ways of the past.
Omo stands for what Obi calls the "old African," which is representative of a more submissive, (to the British) older generation of Nigerian. It is a generation that has more "fear" of the British than the younger generation, which longs for independence and freedom.