Preheat the oven to 350 degrees F.
Toast the bread: In a large bowl, add the bread slices and toss with the olive oil, a sprinkle of salt and the oregano. Toss to blend and arrange them in a single layer on a baking sheet. Place the tray in the oven and bake until golden brown, 6 to 8 minutes. Set aside. In a large pot, heat the oil to 375 degrees F. Prepare a baking sheet fitted with a kitchen towel to drain the leaves as they come out of the oil. To test the temperature of the oil, put a few Brussels sprout leaves on a slotted spoon and submerge them in the oil. It should bubble slightly and the leaves should gradually rise to the top. Fry the rest of the leaves in small batches until they become light brown. Transfer them to the kitchen towel. Season with salt immediately.
Assemble the crostini: Top each slice of bread with some of the ricotta. Season with salt and pepper. Top with some of the Brussels sprout leaves and the Parmesan. Sprinkle with vinegar. Serve immediately.