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, 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 3 1/2 cups, 30 to 45 minutes, adding the oregano halfway through. Discard the bay leaf.