Jamie Oliver has recipes from all over the world and is enthusiastic about the cuisine of many countries. He does have a special interest in Italian cookery, having trained with the Italian chef Gennaro Contaldo, and one of his major business interests in Jamie's Italian, a chain of UK restaurants - you can see details here. One of his cook books is called Jamie's Italy and he has shown lots of Italian recipes on TV, so it's clearly one of his top cuisines.
However, it wouldn't be accurate to say that Italian is his speciality, because a big part of Jamie Oliver's style is taking an interest in good cooking wherever it comes from. He has been equally keen to promote traditional British food - Jamie's Great Britain was a big seller - as well as US, Mexican and different Asian cuisines.
So it would be more true to say that he specialises in making all kinds of good food accessible to home cooks - you can get an idea of the variety of his interests here.