Step #1 In a small heavy skillet dry-roast oregano over moderate heat, about 2 mins, shaking skillet every once in awhile, until fragrant & beginning to brown, & transfer to a small bowl.
Step #2 Cool oregano completely & in an electric coffee/spice grinder grind fine.
Step #3 In a heavy skillet heat oil over moderately high heat until hot but not smoking and, using tongs, turning them, 1 or 2 at a time, fry chiles, until puffed & just beginning to brown, about 10 seconds.
Step #4 Do not let chiles burn or rub will be bitter.
Step #5 Transfer chiles as fried to paper towels to drain & cool until crisp.
Step #6 Wearing rubber gloves, break chiles into pieces & in coffee/spice grinder grind fine in batches.
Step #7 In a food processor/blender grind oregano & chiles with garlic & salt until mixture is a shaggy, saltlike consistency.
Step #8 If mixture seems moist, on a large baking sheet spread it into a thin even layer & dry in middle of an oven set at lowest temperature until no longer moist, about 1 hr.
Step #9 Wearing rubber gloves, break up any lumps with your fingers.
Step #10 Chipotle rub keeps in an airtight container, chilled 6 months.