1/3 cup (or more) olive oil
1/4 cup vegetable oil
1 cup pitted olives (such as Niçoise or Kalamata), chopped
1 tablespoon minced capers
1 garlic clove, minced
1 anchovy fillet packed in oil, drained, minced (optional)
Stir 1/3 cup olive oil, next 4 ingredients, and anchovy, if using, in a small bowl. Add more olive oil for a thinner tapenade, if desired. (For a smoother sauce, purée tapenade in a food processor or blender.) Set aside.
Place steak in a large baking dish and season generously with salt. Stir orange zest and juice and remaining 6 ingredients in a small bowl to combine. Spread mixture evenly over both sides of steak and let marinate at room temperature for 1 hour.
Build a medium-hot fire in a charcoal grill, or heat a gas grill to high. Grill steak, turning once, until nicely charred, about 5 minutes on each side for medium-rare. Transfer to a carving board and let rest for 5–10 minutes.
Arrange tomatoes on a serving platter. Scatter shallot and parsley over; drizzle with oil and season with salt and pepper. In a medium bowl, toss watercress with 1 Tbsp. each oil and lemon juice. Season watercress to taste with salt.
Mound watercress on platter. Slice steak against the grain; transfer to platter with tomatoes and watercress. Spoon tapenade on steak and serve alongside.