A massive loss. My thoughts are with his family.
I spoke to him a few times and he was amazingly helpful. Whenever someone asked me about an obscure piece of German gear, my go to line was 'shoot Oliver an email' - I did this many times over the years. As well as being a supremely knowledgeable engineer, the audio community has lost a very important historian.
edit - come to think of it, I don't think I can think of an instance where he couldn't ID or give info about a rare German item. He seemed to know the background history on any vintage item made in Germany.