Cook the couscous according to the package directions.
Heat grill to medium-high. Rub the pork with 1 teaspoon of the oil and season with the oregano, ½ teaspoon salt, and ¼ teaspoon pepper.
In a bowl, toss the broccoli rabe with the remaining 1 tablespoon oil and ¼ teaspoon each salt and pepper.
Grill the pork, covered, turning occasionally, until cooked through, 18 to 22 minutes.
Fifteen minutes into the pork’s cooking time, place the broccoli rabe and lemons on the grill. Cook, turning, until the broccoli rabe is tender and the lemons are charred, 6 to 7 minutes.
Fluff the couscous and mix in the pistachios. Slice the pork and serve with the broccoli rabe and lemons.