Heat a grill or grill pan over medium-high heat.
Season the chicken breast with salt, black pepper, and garlic powder, then coat it with the teaspoon of olive oil.
Grill the chicken for 5-7 minutes on each side, or until fully cooked.
Remove the chicken from the heat and let it rest for a minute before slicing it.
In a small bowl, whisk together the lemon juice, olive oil, salt, black pepper, and oregano to make the dressing.
In a large bowl, place the chopped lettuce and add the cucumber, tomatoes, and red onion.
Top the vegetables with the sliced grilled chicken and avocado cubes.
Drizzle the dressing over the salad and toss gently just before serving.