Season the chicken breast evenly with half of the dried oregano, sea salt, and black pepper.
Heat 0.25 tablespoons of olive oil in a skillet over medium-high heat and sear the chicken for 6 to 7 minutes per side until fully cooked and golden.
While the chicken cooks, chop the romaine lettuce into bite-sized pieces and halve the cherry tomatoes.
Slice the cucumber into half-moons and finely dice the red onion.
In a small glass jar or bowl, whisk together the remaining olive oil, lemon juice, and the rest of the oregano, salt, and pepper to create the vinaigrette.
Combine the lettuce, tomatoes, cucumber, onion, and kalamata olives in a large bowl and toss gently.
Slice the rested chicken breast into strips, place them over the salad, sprinkle with crumbled feta cheese, and drizzle with the prepared dressing.