For me, this has always been an iconic modernist logo. According to this correspondence looking for brand info, the logo was designed in-house by the Head of Design in 1969.
Whilst the brand guidelines aren’t available,this flickr set is a Fascinating peek at the extensive document.
The original, flat logo is an elegant combination of the letters creating a versatile and lasting brand. As covered on Daivid Airey’s logo love there is also a subtle nod to the historical use of shields in other university branding. I’m not sure what the addition of the gradients and shadow to the logo are meant to achieve. For me. they are just modish distractions from a basic classic.
Also worth mentioning is the University’s misson for distance learning meant that there was a very strong motion graphics component to the brand. Generations grew up with the Open University idents on day time and late night television as seen in this youtube video
Finally, I can’t mention the Open University without linking to this Fry and Laurie sketch.