That's my basic problem:clearly show the difference in recipes. BJCP says "A deeper malt character distinguishes these beers from Flanders red ales. The Oud Bruin is less acetic and maltier than a Flanders Red, and the fruity flavors are more malt-oriented." I'm not too familiar with this style, Flemish red is much more common where I live. So I'll have to do some extensive tasting when I get back from vacation.
I'm not sure the actual recipe distinguishes the styles as much as fermentation drives a difference. Modern Belgian oud bruins are also often just lacto and sacc so they don't dry out as much and more malt sweetness remains to oxidize into sherry-like fruit flavors with time. They tend to be stainless aged rather than oak aged like reds. You can find other unusual processes like Petrus oud bruin which is a blend of a sour pale with a clean brown ale and IMO tastes more like a red than an oud bruin like Goudenband but not quite that sweet-sour of a backsweetened red like Monk's Cafe or Duchess.