Prepare grill for cooking. If using a charcoal grill, open vents on bottom of grill and on lid.
Squeeze enough juice from oranges to measure 5 tablespoons. Mash garlic to a paste with 1/2 teaspoon salt using a mortar and pestle (or mince and mash with a large heavy knife), then whisk together with orange juice, 2 tablespoons oil, and 1/2 teaspoon oregano.
Pat pork dry and rub each tenderloin with 1/2 tablespoon oil, 1/2 teaspoon salt, 1/4 teaspoon pepper, and 1/4 teaspoon oregano.
When fire is hot (you can hold your hand 5 inches above rack for 1 to 2 seconds), grill pork on lightly oiled grill rack, covered with lid, turning once, until an instant-read thermometer inserted diagonally into center of each tenderloin registers 155 degrees F, 12 to 14 minutes total. Let pork stand 5 minutes before slicing.