In a small bowl, whisk together the oil, lime juice, chili powder, cumin, garlic, lime zest, jalapeno, oregano, salt, black pepper, and red pepper flakes.
Place the chicken or beef in Ziploc bag with the fajita marinade. Seal the bag and make sure all of the pieces are well coated. Place the bag in the refrigerator for at least 1 hour and up to 8 hours.
When ready to cook, preheat the grill to medium-high heat. Thread marinated meat, onion, and peppers onto skewers.
Lightly oil the grill grate. Place the kabobs on the grill, cover, and cook about 8-12 minutes, turning once or twice during cooking.
Transfer to a warmed platter and serve immediately.