I know Romania was the major oil producer in Europe before WW2, and that Germany deserted the oil fields during the war, but I don't understand how they weren't able to get rich off the oil and become a 2nd degree power in Europe, and maybe the world before that happened and before the Gulf countries took over the oil market.
I know this is one month after you posted this question so I'm not even sure if you'll see this, but:
Romania did get rich off the oil it sold to various European countries. In fact, pre-World War II Romania is undoubtedly a second degree power in Europe on almost every level - be it economically, politically or militarily. In 1938, the Romanian economy was one of the most well-off in Europe, definitely behind the Soviets, British, Germans, French, Italians but leagues ahead of countries like Spain, Portugal, Sweden or Hungary.
However, Romania's antebellum wealth is not very apparent at first glance because the average wealth of the country was low, even for 1938 Europe standards. The rising class of capitalists was indeed obscenely rich (and you can still see that today through some of the absolutely extravagant/stunning pre-war villas in parts of downtown Bucharest), but the middle class wasn't very numerous and most Romanians kept living an agrarian lifestyle and were nothing more than subsidence farmers. This isn't to say that they didn't live well - they did, but the average man/woman didn't benefit off the production/sale of oil as well the capitalists and the bourgeoisie did.