To prepare tomatoes: Preheat oven to 300 degrees F. Coat a large rimmed baking sheet with cooking spray. Cut tomatoes in half lengthwise and place on the prepared baking sheet. Sprinkle each half with 1/8 teaspoon garlic oil and some oregano and salt. Roast for 2 hours. When cool enough to handle, coarsely chop tomatoes and transfer (with juices) to a serving bowl.
To assemble tapas: Shortly before serving, preheat oven to 350 degrees F. Place bread on a baking sheet and brush with some of the remaining garlic oil. Bake until slightly crispy, but not hard, 2 to 4 minutes per side. Let cool slightly. To serve, arrange the bread on a large platter with Serrano ham and the bowl of tomato mixture for spreading.