Preheat the grill or grill pan over medium heat.
In a small bowl, whisk together olive oil, balsamic vinegar, dried oregano, salt, and pepper. Set aside.
Lightly coat the grill grates with oil to prevent sticking. Place the halloumi slices on the grill and cook for about 3-4 minutes on each side, or until golden grill marks appear and the cheese is warmed through.
While the halloumi is grilling, prepare the salad. In a large bowl, combine the mixed greens, cherry tomatoes, cucumber, red onion, and olives.
Once the halloumi is done, remove it from the grill and let it cool for a minute before cutting into smaller pieces if desired.
Drizzle the dressing over the salad and toss gently to combine. Top with the grilled halloumi cheese.
Serve immediately on individual plates.