Preheat oven to 450°F. Coat a baking sheet with cooking spray.
Rub cod with 2 teaspoons oil. Sprinkle with pepper. Place on the prepared baking sheet. Transfer to the oven and roast until the fish flakes easily with a fork, 15 to 20 minutes, depending on the thickness of the fillet.
Meanwhile, heat the remaining 1 teaspoon oil in a small skillet over medium heat. Add shallot and cook, stirring, until beginning to soften, about 20 seconds. Add tomatoes and cook, stirring, until softened, about 1 1/2 minutes. Add olives and capers; cook, stirring, for 30 seconds more. Stir in oregano and vinegar; remove from heat. Spoon the tapenade over the cod to serve.
Ingredient note: Cod: Overfishing and trawling have drastically reduced the number of cod in the Atlantic Ocean and destroyed its sea floor. A better choice is Pacific cod (aka Alaska cod); it is more sustainably fished and has a larger, more stable population, according to Monterey Bay Aquarium Seafood Watch (www.mbayaq.org/cr/seafoodwatch.asp).