A challenging aspect between two planets
Aspect angle: 180 degrees
Unfavorable. When two planets form an opposition, they are directly opposite each other in the zodiac, and their energies are working against each other. This is a stressful planetary aspect, yet still not as harsh as some other hard aspects, because planets in opposition at least have the same intention. So even while the planets are at odds and there are two sides to the story, they may still be able to reach a compromise.
Take Mars and Uranus: When the planet of action and the planet of revolution are opposite each other, it may result in explosive or impulsive behavior. It may trigger sudden reactions, and you could blow up at someone who's been holding you back. On the positive side, it may also trigger you to break free from unwanted obligations or restrictions.
Planetary oppositions can have positive or negative results, but either way they are really about the relationship between dark and light, content and discontent, certainty and uncertainty. Opposite aspects help us see both sides so we can negotiate the best outcome.