Open the can of tomatoes and pour the juice into a bowl. Use the lid to press against the tomatoes to extract as much juice as possible. Put the tomatoes in a separate bowl, then use your hand to squeeze the tomatoes to a pulp. Reserve the juice and pulp separately. Fill the empty can half full with water and set aside.

Heat the olive oil in a heavy saucepan over moderately high heat. Add the onion and cook, stirring occasionally, until soft, about 2 minutes. Add the garlic and cook until golden. Add the tomato juice and bring to a boil. Simmer rapidly until the juice thickens, then add the crushed tomato pulp, the half can of water, the bay leaf, and salt and pepper to taste. Adjust the heat to maintain a simmer and cook, stirring occasionally to prevent scorching, until the mixture thickens and reduces to about 3½ cups, 30 to 45 minutes, adding the oregano halfway through. Discard the bay leaf.