Oppositions... The way I see it:
Imagine X and Y are two planets in opposition.
When you express planet X, you'll be met with a reaction that is characteristic of planet Y, from the external world (person or event).
If you express planet Y, you'll be met with a reaction that is characteristic of planet X.
And that dynamic reflects an inner opposition in yourself - two sides of an issue that you have trouble balancing.
That is why, the opposing reaction can also come from yourself and not necessarily manifest as an external event.
Ex:
Venus opposite Uranus
When you express Venusian traits, like affection, the desire for closeness and relationship, you may either feel an internal feeling of unease - Uranus trying to "break away" from the closeness, to regain its desire for freedom; or you may be faced with someone who acts distant / cold - playing out the Uranus energy for you.
Conversely, when you act distant / non-commital, a part of you may feel the need for closeness ("I'm feeling lonely now") or you may be faced with someone who acts clingy / needy with you and demands your attention / love / affection (Venus).
There is *usually* less obvious tension in the opposition as opposed to the square, which is more internal and more under pressure.
With the opposition it's like a back and forth between one energy and the other.