In a small mixing bowl, combine the olive oil, soy sauce, lemon juice, brown sugar, Dijon mustard, garlic, dried oregano, salt and pepper. Whisk to mix. If desired, reserve 2 tablespoons of marinade in a separate small dish to brush on the pork partway through cooking.
Trim any silver skin off of the pork tenderloin. Place the pork in a large zip-top plastic bag. Pour the marinade into the bag with the pork and seal.
Marinate in the refrigerator for at least 1 hour and up to 12 hours or overnight.
After marinating, you can cook the pork tenderloin on the grill, in the oven or in the air fryer.
To grill: Heat grill to medium heat. Grill the pork tenderloin (discard any used marinade) over medium heat with the grill closed for 15-25 minutes, turning every few minutes, until the center of the tenderloin reaches 145° F. If desired, brush pork with the reserved 2 tablespoons of marinade when it has just a few minutes left on the grill. Let pork rest for 5 minutes before slicing and serving.
To bake: Preheat oven to 400° F. Heat 1 tablespoon of olive oil in an oven-safe skillet over medium-high heat. (If you do not have an oven-safe skillet you can transfer the pork to a baking dish after browning it.) Add the pork to the pan, discarding the extra marinade. Brown on all sides, 1-2 minutes per side. Transfer to the oven and bake for 20-25 minutes, or until the internal temperature of the pork reaches 145° F. Let rest for 5 minutes before slicing and serving.
To cook in the air fryer: Preheat the air fryer to 380° F for 5 minutes. Air fry the pork at 380° F for 10 minutes. Then flip the tenderloin over and continue cooking for 8-16 more minutes, or until the center of the pork registers 145° F on an instant read thermometer. Let it rest for 5 minutes before serving.