Otto Franz Joseph Karl Ludwig Maria, Prince Imperial and Archduke of Austria, Prince Royal of Hungary and Bohemia (21 April 1865 – 1 November 1906) was the second son of Archduke Karl Ludwig of Austria (younger brother of Emperor Franz Joseph I of Austria) and his second wife, Princess Maria Annunciata of Bourbon-Two Sicilies. He was the father of Charles I of Austria, the last Emperor of Austria.

Otto was a son of Archduke Karl Ludwig of Austria and his wife, Princess Maria Annunciata of Bourbon-Two Sicilies. Otto’s father, Karl Ludwig, was a younger brother of Emperor Franz Joseph I of Austria; and Karl Ludwig became heir presumptive to the Austro-Hungarian throne when his nephew Crown Prince Rudolf died in 1889. Although a newspaper account claimed that Karl Ludwig renounced his rights to the throne that same year (1889) in favour of his eldest son, Franz Ferdinand that story is not true.
On the death of Karl Ludwig in May 1896, Otto’s brother Franz Ferdinand did indeed become heir presumptive to the Austro-Hungarian throne. At the time of their father’s death, Franz Ferdinand had been ill with tuberculosis and there was speculation that Franz Ferdinand would renounce his rights, which would have made his brother, Otto, heir presumptive. However, this did not happen, and Otto was never first in line to the throne.
In 1914, Franz Ferdinand was murdered in Sarajevo, and Otto’s son Charles became heir presumptive. Charles inherited the throne two years later.

Otto’s mother died when he was six years old. Otto and his elder brother Franz Ferdinand were educated by Alfred Ludwig, Baron of Degenfeld. Otto was not interested in learning and often played pranks on his teachers. Nevertheless, his teachers liked the cheerful Otto better than his grumpy and irascible older brother. He was also his father’s favourite, which led to a difficult relationship with his brother.
Otto had a reputation as a loafer and was often involved in scandals. He was gradually alienated from the imperial court, and eventually even his wife distanced herself from him.

Around 1900, he contracted syphilis. This caused him agonizing pain for the last two years of his life. He withdrew from public life and spent a year in Egypt, where he found temporary reprieve. He returned to Austria, where he fell ill again. The last few months of his life, he lived in a villa in the Viennese suburb Währing. He was seriously ill, and was nursed by his last mistress, Louise Robinson, using the pseudonym Sister Martha, and by his stepmother Infanta Maria Theresa of Portugal (1855-1944). He died on 1 November 1906, in the presence of his spiritual adviser, Godfried Marschall, the auxiliary bishop of Vienna.