Cut squash in half lengthwise and remove the seeds. Place both halves cut side up on a baking dish and roast for about 45 minutes or until the fork can easily puncture the flesh.
Using a grill pan or countertop grill, cook chicken breast.
While chicken breast is cooking, heat olive oil in a small pan. Add garlic, tomatoes, and oregano, and cook for about five minutes.
When spaghetti squash is ready, remove strands from the skin, place in bowl and top with tomato mixture and grilled chicken breast.