Remove stems and seeds from chiles; coarsely chop. Place chiles in a spice or coffee grinder; process until finely ground to measure 1/4 cup.
Heat oil in a medium saucepan over medium heat. Add onion and garlic to pan; cook 3 minutes or until onion is tender, stirring frequently. Add ground chiles; stir well. Stir in 1 cup chicken broth. Add the remaining 1 cup chicken broth, 2 cups water, dried oregano, and salt; bring to a boil. Reduce heat, and simmer until reduced to 3 cups (about 20 minutes), stirring occasionally.
Sprinkle masa harina over chile mixture; stir well. Bring to a boil. Cook for 1 minute or until slightly thick; remove from heat. Let stand 10 minutes. Place half of the chile mixture in a blender. Remove the center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Process until smooth. Pour into a large bowl. Repeat procedure with the remaining chile mixture.