Cook pasta shells in a large saucepan of lightly salted boiling water until al dente, following packet instructions.
Heat half the oil in a large non-stick frying pan over medium-high heat. Add prawns and one-third of the garlic. Sauté until prawns just turn pink and curl, 2 to 3 minutes. Transfer to a plate.
Heat remaining oil in the same pan. Add remaining garlic, the anchovies and chilli flakes. Sauté until garlic has softened, about 30 seconds. Do not let it brown. Add the tomato and oregano. Sauté until tomato is softened, 3 to 4 minutes. Add cooked prawns, olives and capers.
Drain pasta well. Toss with prawn and tomato mixture. Sprinkle with black pepper.