I understand some came from Romania, but that could have provided only a small portion of the oil needed.
About one third to one half of the oil was refined synthetically from coal. The German term for this kind of refinery plant is "Hydrierwerk". There's a fairly exhaustive article in the German Wikipedia about it, but unfortunately it hasn't been translated to other languages:
The article says:
- At the beginning of the war 7 refineries were active (the biggest in Leuna), 3 more were nearly finished.
- In 1943 twelve refineries existed, in 1944 fifteen.
- The production had a maximum in 1943. Due to heavy bombing most of the capacity was destroyed until March 1945.
The Wikipedia article contains a table with production numbers. For example, in 1942 Germany consumed 9,500,000 t (estimated), and produced 4,920,000 t from coal.
There is a theory that only after dividing Romania, with its oil fields, and ceding a portion of it to Stalin did Hitler realize that he would soon run out of oil. The Battle of Stalingrad was Hitler's desperate attempt to get to the oil-rich Baku. Stalin and the Allies knew that once Hitler had access to those fields, there would be no stopping him, which was why the battle turned out to be so gory. After the failure at Stalingrad, it was all downhill for him. Converting coal is an extremely expensive, labor-intensive process. Severe shortages followed; Hitler was doomed.
It's only a theory, though.