Oppenheimer was born in Germany and studied art even before he was old enough to enrol at Munich Academy of Art. Eventually at 16 he was admitted to formally study at the Academy. He then went on to study in Rome, London and New York. He settled in the US for sometime where he met John Singer Sargent and James Whistler. He earned a reputation as a modern impressionist portrait painter and took on many commissions in Germany and abroad.
He moved back to Germany when he got married but continued to spend time in London where he was particularly fond of the Thames and often stayed in Henley during his visits. He later moved to Britain permanently and gained his British Citizenship in 1939. From 1934 to 1965 he was a member of the Royal Society of Portrait Painters in London, where he produced works adapted to the British impressionist style that were accepted for exhibition on a regular basis. Though there are some key pictures in public collections, the vast majority of his paintings are held in private hands.