Bring a large pot of salted water to a boil. Cook the rotini until al dente according to package directions, about 8 minutes. Drain and rinse with cold water.

In a small bowl, whisk together olive oil, red wine vinegar, lemon juice, Dijon mustard, garlic, oregano, salt, and black pepper until emulsified.

Dice the bell pepper, cucumber, and red onion; halve the cherry tomatoes; slice the olives; and crumble the feta. Keep the avocado aside.

In a large bowl, combine the cooled pasta with all the vegetables, olives, and feta. Pour the dressing over the top and toss gently to coat.

Cover and refrigerate for at least 20 minutes. Just before serving, fold in the diced avocado, toss again, and adjust seasoning with salt and lemon juice if needed. Serve chilled or at room temperature.