Preheat the oven to 350 degrees F.
In a medium bowl, toss the squash with the sugar and half of the olive oil. Season with salt and stir in the oregano. Arrange the squash in a single layer on a baking sheet and place it in the center of the oven. Cook until the squash is tender but still holds its shape and is somewhat firm, 15 to 20 minutes.
In a small bowl, combine the red wine vinegar and the raisins. Set aside.
Heat a medium skillet and add a little of the remaining olive oil. When it begins to smoke lightly, add the onions and garlic. Season with salt and cook until they are translucent, 2 to 3 minutes. Transfer to a bowl and stir in the tomatoes, celery, capers, and olives. Stir to blend. Refrigerate.
When the squash is cooked, remove from the oven and allow it to cool. Stir it into the onion mixture. Stir in the raisins and vinegar. Taste for seasoning. Refrigerate until ready to serve.