Slice eggplant, zucchini, and red onion into 1/2-inch thick rounds.

Cut bell peppers into quarters, removing seeds and membranes.

In a large bowl, whisk together olive oil, lemon juice, minced garlic, oregano, salt, and pepper.

Add the vegetables to the bowl and toss to coat evenly.

Preheat grill to medium-high heat.

Place the vegetables on the grill.

Grill for 10-15 minutes, turning occasionally, until tender and lightly charred.

Remove from grill and arrange on a serving platter.

Drizzle with any remaining marinade from the bowl.

Serve warm or at room temperature.