Arrange a rack in center of oven; preheat to 350°. On a baking sheet, toast walnuts, stirring halfway through, until darkened and fragrant, about 5 minutes. Let cool, then coarsely chop.
Meanwhile, in a medium bowl, combine peaches, cucumbers, tomatoes, blackberries, olives, basil, mint, oil, lemon juice, and walnuts; generously season with kosher salt and pepper and gently toss to combine.
Spread ricotta on a platter or divide among plates. Drizzle with honey and top with lemon zest; season with sea salt.
Spoon peach salad over ricotta. Top with cheese and oregano. Drizzle with more oil.
