Fernando Poo is an island 32 km off the west coast of Africa, specifically Cameroon, in the Gulf of Guinea. Much of the action of the trilogy is centred around a coup d'etat on the island which is a trigger for nuclear war.
In the book the name of the island is changed from Po to Poo, why this is is not certain, it could be down to a spelling error. The relevance of the original spelling would indicate that it should be kept as Po because the I-ching Po (23) is a signifcant symbolic part of the book, indicating the increasingly chaotic political circumstances of the plot, it could be that adding the extra 'o' brings attention to the significance of the word itself... however, it is most likely just toilet humor.
The Portuguese navigator Fernão do Pó (the first European to visit the island) in 1472 named it Formosa Flora ('Beautiful Flower'), but in 1494 it was renamed by Europeans as Fernão do Pó. The island's native name (in the Bube language) is Otcho. The modern name of the island is Bioko.