Pre-heat oven to 400 degrees and line a baking sheet with foil. Spray with cooking spray then place salmon on top. Drizzle with olive oil then sprinkle with salt, pepper and oregano. Bake until it flakes easily with a fork, about 15-20 minutes; set aside.
While salmon is baking, make the Greek Tzatziki Sauce in a medium bowl; set aside. Next, make the Greek Vegetable Tapenade by mixing together all ingredients in a medium bowl. Toss well then adjust seasoning accordingly; set aside.
To assemble, place the tostadas on a plate and spread a couple tablespoons of tzatziki sauce on each. Next, top with flaked salmon then top with a couple tablespoons of the Greek Vegetable Tapenade. Garnish with thinly sliced jalapeños if you wish for a little kick. Serve immediately.