In a large mixing bowl, combine the ground chicken, crumbled feta, chopped spinach, red onion, oregano, garlic powder, salt, and pepper.
Gently mix the ingredients by hand until just combined, being careful not to overwork the meat to maintain a tender texture.
Divide the mixture into 4 equal portions and form each into a patty approximately 3/4-inch thick.
Heat the olive oil in a large non-stick skillet or cast-iron pan over medium heat.
Place the patties in the skillet and cook for 5 to 6 minutes per side, or until the internal temperature reaches 165°F (74°C) on an instant-read thermometer.
Remove the burgers from the heat and let them rest for 3 minutes before serving on buns or in lettuce wraps.