Philip Johnson's Gate of Europe in Madrid, seen here, which are like any standard, Modernist-esk office building, only they are pitched in toward each other at 15 degree angles.
Koolhaas' builing is remarkable. From the outside it looks as if the elevated train had fallen and crushed this flat two story building and they just left it. With the two triangular forms on either side crashing into each other under the heavy train tunnel. Inside, the one form juts out into the negative space of the other, The metal clad tunnel is seen through a reveal in the ceiling. The form were kept three separate entities (train, public space, private space) yet brought together to create one unified space. The building squishing below the track instead of merely being controlled by it show the embrace of the existing environment, as does the integration to the old IIT cafeteria designed by Mies. The grid of I-beam structure is also continued in the new area, mixed with the new structure.
The Modernist were trying to create an international style by taking out all history and context. The Post-Modernist like Koolhaas succeeded in creating an international style by pouring handfuls of history, context, and style together to create a unified mash-up of form and styles. In this respect, when everything is in one, anything can belong in the space, and the style can change from building to building, it can be whatever is necessary for the site. In my opinion, the Post-Modernist succeeded in creating an international style by making it possible to make buildings in the shape of books or pianos. They succeeded in a much different and much more open and fun-loving way than the Modernists.