Cook garlic in oil in a deep 4 to 6-quart heavy pot over moderately low heat, stirring, until garlic is fragrant, about 1 minute. Stir in tomatoes and 1/2 tablespoon oregano and cook, stirring occasionally, until tomatoes break down, 4 to 5 minutes. Season with pepper.
Add clams, stirring well to coat, then cover pot tightly, and increase heat to moderately high. Cook, stirring once, until clams open, 5 to 7 minutes. (Discard any clams that have not opened after 7 minutes.)
While clams are cooking, toss together bread crumbs, zest, and remaining 1/2 tablespoon oregano. Divide clams and pan juices among 4 shallow bowls, then sprinkle with bread crumbs.
