Me and my partner are completely opposite and is exactly with cancer and capricorn like you gave examples.

He is sun, venus, mercury and MC in Cancer.

I am moon, venus, jupiter and MC in Capricorn.

Our Mars are also opposed, his is in leo, mine in aquarius.

Our ascendants are opposed too exactly.

And my ascendant is conjunct his north node and moon and jupiter.

So you have a very good point. I have this flashbacks with him where I know things even before he tells me. I know he was the one even before we met personally. I even guessed the day of his birthday when was his birthday.

But we are very different. Very. But is complementary; sometimes we butt heads strongly, but after some hours we forgive and try to meet halfway.
We mostly help each other in areas we lack so much we wouldn’t be able to see if we were alone, simply because we lack that vision, we cant see it, and that could make our lives poorer and unhappier. I could end up alone, a control freak, without a partner, without family, without knowing what home is and old, without spiritual vision. Mathematical and without emotions, maybe even ending depressed in old age.
He could lose his house because he forget about practical details, about saving, about common sense on financial and practical matters. Never progressing in career because of being too afraid to change or try to advance and end up being a stay at home dad, but frustrated and depending on someone else. Or not having someone to talk too because of wanting to hide in his hermit shell (he even thought of being single forever as an option, before we met).
So, I think is something very good. And sometimes, with people with conjunctions is too strong. Too much of the same thing and although I feel understood, it tires quickly and makes you feel annoyed in the long run. They are best as friends, but lovers I don’t know. Even the concept of female and male. Is an opposition in its essence. Men are square usually, women are full of curves, and so on.So in love I think oppositions (but depending on which planets) can be very rewarding but also a testing path along the way. Kudos for your perceptions!

I have found oppositions to be very interesting in people too! I had a roommate who found it so difficult to share her things. when I looked at her chart I saw that she had inner planets in 2nd opposite Outer planets in 8th.

What should one know about a Leo Sun opposite Aquarius Saturn? ( It’s a lone angular opposition with no squares, trines, sextiles to it- ease, negotiating aspects that is.)In this case what is it that has to be reconciled?

Most astrology sites name the result of having aspects between planets( in this case- loser, negative, critical) not the process itself that undergoes so that one can come up with the positive and negative results themselves.

I will do a bit of research and see if I can come up with any history on the establishment of the octaves. If I can, I’ll explore this further.

how is that defined?

at first I thought it was because the traditional planet rulerships of the signs were perhaps the lower octave of the higher planets (due to Mars-Pluto for Scorpio) but then I remembered that Jupiter is the traditional ruler of Pisces (Neptune) and that Saturn is the traditional ruler of Aquarius (Uranus).