Segment the oranges, removing both the pith and the skin, leaving only the flesh and then cut into slices. Slice the chilli into rings and finely chop the basil leaves and garlic. Heat the oil in a frying pan, add the garlic, chilli and prawns, and fry for a few minutes. Meanwhile, cook the spaghetti in salted, boiling water. Strain and toss together with the other ingredients before serving.