The Oca river, the Ebro's first tributary Nature · Rábanos, Montes de Oca, Burgos Image of Rábanos, not of the place itself. The Oca rises in the sierra above Rábanos and comes down from south to north over seventy kilometres as far as Oña, where it delivers its waters to the Ebro: it is the first tributary that the great river receives from its source. Photo: Thomas Holbach (CC BY-SA 4.0) What to see and do in Rábanos