Prepare the Chicken:
Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Pat chicken breasts dry. Drizzle with 2 tbsp olive oil, oregano, 1/2 tsp salt, and 1/4 tsp black pepper. Rub to coat evenly.
Bake for 20-25 minutes, or until internal temperature reaches 165°F (74°C). Let cool completely, then dice or shred.
Prepare the Vinaigrette:
In a small bowl, whisk together 1/4 cup olive oil, lemon juice, red wine vinegar, 1 tsp oregano, Dijon mustard, 1/4 tsp salt, and 1/8 tsp black pepper until well combined.
Assemble the Salads:
Divide the mixed greens evenly among 4-5 meal prep containers.
Top each container with an equal portion of diced chicken, Kalamata olives, crumbled feta, cherry tomatoes, and sliced red onion.
Store the vinaigrette separately in small dressing containers.
When ready to eat, drizzle the dressing over the salad and toss to combine.