Boil the spaghetti in a saucepan in plenty of lightly salted water until al dente.
Peel and finely dice the onions and garlic cloves. Pluck the oregano leaves and only chop half of them. Drain the capers and olives and cut the olives in half or into slices, depending on their size.
In a large, wide saucepan, heat the olive oil and sauté the onion and garlic cubes and the chopped oregano over a low to medium heat for about 2 minutes. Then add olives and capers and cook for about 1 minute. Then add the chunky tomatoes and cover everything and simmer over a low heat for about 15 minutes. Season the sauce with a little salt, a few turns of colored pepper from the mill and a pinch of sugar.
Drain and fold in the spaghetti. Arrange on plates and sprinkle with the finely diced feta and the remaining oregano leaves.