Cut the tenderloin in half width-wise.
In a large sealable plastic bag, whisk together the orange juice, soy sauce, garlic powder and brown sugar.
Add the two tenderloin pieces to the bag, squeeze out any air and seal the bag shut. Move the tenderloin pieces around in the bag so that they are well coated in the marinade. Refrigerate the bag for 30 minutes and up to 1 hour.
When ready to cook, remove the tenderloin pieces from the bag and pat them dry with paper towels.
Add the vegetable oil to a large cast-iron or stainless steel skillet set over medium-high heat. Once the oil is hot, add both of the tenderloin pieces to the skillet. Let them cook, undisturbed, until they brown and release naturally from the skillet, about 5 minutes. Flip the tenderloin pieces and continue cooking them on all sides until the pork reaches an internal temperature of 145°F.
Remove the tenderloin pieces from the skillet and let them rest for 5 to 10 minutes then slice and serve.