I just did a quick scan of my "library" for you, and I found one book that says that Oldenburgs are primarily bay, black, brown, or grey, but did not exclude any other color. Another book allows for all solid colors. As Nutty Saddler said, with a background so diverse it would not surprise me to see a buckskin or dun. Every horse breed has "recommended" colors, but that doesn't mean that other colored foals aren't born. There are (rarely) bay Lippizaners, and through my own Googling I have found red chestnut and even a grey Friesian.
Here's one buckskin Oldenburg pic I found: http://www.enavantstud.com/images/gold.jpg