Whisk 3 tablespoons olive oil, 2 tablespoons fresh lemon juice, oregano, and minced garlic together in a large bowl. Generously season with salt and black pepper, to taste, and stir to combine. Set aside.
Place the cubed chicken in a large bowl and pour half the marinade on top. Toss to combine and set aside.
Add the sliced zucchini and yellow squash to the original bowl with the marinade and toss to combine. Set aside.
Place a large nonstick skillet over medium heat. Once hot, add the remaining tablespoon olive oil and the sliced red onion.Season with salt and black pepper, to taste, and stir to combine. Cook, stirring occasionally until the onion softens and start to develop some color, approximately 4-5 minutes.
Pour the seasoned chicken into the hot skillet and cook, stirring occasionally, until the chicken is golden brown and cooked through, approximately 8-10 minutes. Remove from heat and transfer the chicken and onions to a platter. Set aside.
Increase the heat to medium-high and add the seasoned zucchini and yellow squash to the skillet. Season with additional salt and black pepper, if desired. Cook, stirring occasionally, until crisp-tender and nicely browned, approximately 3-5 minutes.
Return the chicken, onions, and juices to the skillet and stir to combine. Remove from heat and sprinkle with remaining fresh lemon juice, chopped parsley, and springs of fresh basil. Serve immediately with additional lemon wedges, if desired. Enjoy!