I have been pondering this question myself lately, and I have a theory to why Oscar is a common name in Spanish speaking countries. As a very young child I assumed my dad's name was somehow Spanish because his name Oscar. It turned out that he is actually Swedish, and Oscar or Oskar is an even more common name in Sweden and common throughout all Germanic countries. Anyways, I looked into this, and this is what I came up with...
In the early 6th century Rome was invaded by Germanic tribes, one of the major tribes being the Goths (who some believe to originate from what is now Götaland of southern Sweden), at some point the Goths became two separate tribes Ostrogoths and Visigoths. The Ostrogoths staying to rule Italy and establishing the Ostrogothic Kingdom, and the Visigoths who went on to conquer Hispania (Spain and Portugal). The Visigoths remained the rulers, and because of this, many Germanic names were used. I believe this is why the name Oscar is continued to be used to this day as a Spanish name, but not really seeming like it.
(But really this is all just speculation, it could have just popped up as a name all on its own in Spain. I apologize for the uncertainty, but I can't find any sources directly stating the origin of the Spanish name Oscar. If someone actually knows feel free to correct me.)