Just behind Oxford Circus (Nike and topshop side by the tube exit) is the Market Place which has a few restaurants like Strada and Carluccios as well as an Apostrophe. Popular for business lunchers so service is normally speedy.
Charlotte street has more individual restaurants.
There is also a cute little Italian cafe on Great Titchfield street but the name escapes me, sorry! It's been a few years since I worked there.