Mash garlic, salt, and peppercorns into a paste, using a mortar and pestle, or run through the food processor adding a little orange juice if needed to make into a paste. Add to blender & stir in sour orange juice, onion, and oregano. Let sit at room temperature for 30 minutes or longer.
In a saucepan, heat olive oil to medium hot (approximately 220 degrees F) and remove from heat. Carefully whisk in the garlic-orange juice mixture (prepared above) until well blended. (I prefer to add the warmed oil to the blender, slowly, in order to make an emulsion. That will make for better coating of the meat)
Marinate overnight for best results. Grill meat until done and serve on tortillas with mango salsa.