Scott Burns, emeritus at Portland State and a student of Peter Birkeland, wrote an excellent article about the OSO Slide. I made a few animations and armchair interpretations about this slide shortly after it happened (here).
Scott discusses the geologic history and how heavy rainfall was a likely co-conspirator that led to the triggering of this horribly destructive landslide. Here is the article.
This is a general location map:
Here is an animation from David George and Dick Iverson at the USGS:
Here is the geologic mapping done at the 1:24,000 scale published by the Washington State Department of Natural Resources. Click on the map below to see the entire map. Below is a clip of the map in the region of the OSO Landslide of 2014. Click here to see this inset map in a new browser window. Note the location of a dotted strike slip fault in the region of the OSO slide.