Cook the chicken: Preheat the oven to 425°F. Add the chicken thighs to a sheet pan and toss with olive oil, oregano, garlic powder, salt, and pepper. Roast for 20–25 minutes, or until the thickest part reaches 165°F. Let rest for 5 minutes, then slice.
Quick-pickle the onion: Add the red onion to a bowl with 1 tablespoon red wine vinegar and a pinch of salt. Toss and let sit for 10–15 minutes.
Make the artichoke salad: Add the artichokes, sun-dried tomatoes, capers, olive oil, and remaining tablespoon of red wine vinegar to the pickled onions. Season with salt and pepper and toss to combine.
Make the balsamic mayo: Mix the mayonnaise, balsamic vinegar, oregano, salt, and pepper until combined.
Prep the bread: Slice the ciabatta rolls in half and lightly toast them if desired.
Assemble: Spread the balsamic oregano mayo generously over the ciabatta. Layer with sliced chicken thighs, fresh mozzarella, and plenty of the artichoke salad, draining off any excess liquid before adding it.
Serve: Close the sandwiches, gently press them together, and slice in half before serving.