What is obsession? It's a pattern of thought. It's a merry-go-round that just won't stop and let you off. It feeds on itself, grows stronger with every repetition.
It is self-defeating behavior. Narrow focus is implied. There is an inability to let go, to release.
6th house dynamic as well as 12th. Routines, repetitions, daily activity. 6th house "afflicted" by Pluto or Saturn might indicate this type of obsession if it touches on personal planets/points.
Can we call "obsession" "fixation"? Then a preponderance of Fixed signs just might be an indicator.
How about the 12th as residence of "self-destructive and self-defeating behaviors" and "repressed contents"? intercepted along with the 6th house - "inaccessible subconscious/conscious energies"
How about Moon as "knee-jerk reactions" and "conditioned responses"...clinging, not letting go.... Moon ruled by Saturn in the 6th, square Pluto in the 6th
Mercury-Pluto contacts Mercury in Leo (fixed) square Pluto
And more....Neptune rules ghosts and goblins and things that go Boom in the dark. Neptune in 8th, ruled by Saturn in the 6th
Maybe retrogradation or interception could signify obsession. Mercury, Mars in the 8th (ruled by Saturn in the 6th), and Saturn are all Rx
A chart pattern such as the bundle or the bucket might show such a perversion of focus. Or even a stellium.
I have almost all of those things (red for specifics). You cad.
I don't think obsession is necessarily always a negative thing. It is just a consuming focus that can just as easily be creative as destructive.
I go back and forth on this. I don't have OCD either, but I was at the very least borderline as a kid (depending on who you talked to). It is hard to find the line between "creative" and "destructive" if obsession is playing part. Although, to be honest, that line is fascinating to me.
As far as synastry goes, I had once in my life an intense attraction with someone. At one point, he was among a crowd of people chatting away, then I walked in. Everyone went dead silent when we locked eyes, as if the entire room became electrified and they all felt it. It was comically serious and became obsessive to the point that a relationship was impossible.
The biggest indicator was his early degree Aqua Mars conjunct my late degree Cap Moon. Individually, my Moon is involved in a T-square, opposite Sun/Merc (Cancer/Leo) and square to Pluto in Scorpio. His Mars is square Pluto, 0 degrees, and that is the only major aspect Mars makes to his other planets. His strong Mars/Pluto square activates my central Sun/Merc, Moon, Pluto T-square.
In addition, his Venus and Jupiter are conjunct in Capricorn, conjunct my Mars in the 8th house. His Venus/Jup ruler Saturn is in Scorpio conjunct his Sun in Scorpio (trine my Sun, exact).
His Chiron is conjunct my AC within 1 degree in Gemini (ruled by Merc). Chiron is the "handle" of his bucket chart, where all his planets span between Libra and Aquarius, starting with Moon and ending with Mars. Then, his Mercury in Sag in the "center" of the bucket is conjunct my Uranus and opposed my Chiron, 0 degrees. That might have added the "electric" quality. His bucket configuration was like a focusing lens on my AC.
So, both of us had indicators of Pluto extremes and Saturn seriousness individually, and it reflected in the synastry with some Uranian kick.