What the opposition is
An opposition is an angular separation of 180° — the two planets sit exactly across the chart from one another, in opposing signs. It is a hard aspect, but of a different flavour from the square: rather than raw friction, the opposition creates polarity and tension between two poles that must be balanced.
Because the two planets face each other, the opposition is the aspect of awareness — we see it most clearly reflected back to us, often through other people.
How the opposition works
An opposition pulls a person between two poles that seem mutually exclusive — independence versus relationship, work versus home. The immature response is to swing between them, or to project one pole onto others and fight it there. The mature response is integration: holding both ends of the axis in a working balance. Oppositions are aspects of relationship and consciousness.
By transit and synastry
By transit, an opposition marks the culmination of a cycle — a moment of full awareness, tension or confrontation, the "full moon" phase of the two planets. In synastry, oppositions create strong attraction and complementarity, but also the classic tug-of-war of relationship. Polarity that both binds and challenges.