In a small bowl, whisk together half of the olive oil, half of the lemon juice, dried oregano, sea salt, and black pepper to create a marinade.
Place the chicken breast in a shallow dish, coat with the marinade, and let sit for 10 minutes.
Preheat a grill or grill pan over medium-high heat and cook the chicken for 6-7 minutes per side until the internal temperature reaches 165°F.
While the chicken cooks, slice the strawberries and cucumber into bite-sized pieces.
In a large bowl, combine the mixed baby greens, strawberries, blueberries, cucumber, and walnuts.
Slice the grilled chicken into strips and place them on top of the salad.
Sprinkle with crumbled feta cheese and drizzle the remaining olive oil and lemon juice over the top before serving.