He was born in
Botoșani, the son of Vlad Onicescu, from
Ștefănești,
Botoșani County, and Ana, from
Oniceni,
Neamț County. He graduated from the Botoșani
A. T. Laurian High School in 1911 with a perfect average grade of 10. That same year, he entered the
University of Bucharest, from where he graduated with degrees in mathematics and philosophy in 1913. From 1914 to 1916 he was a mathematics teacher at the military gymnasium of
Dealu Monastery, near
Târgoviște. From 1916 to 1918 he fought in
World War I. In 1919, Onicescu went to study
geometry at the
University of Rome, under the guidance of
Tullio Levi-Civita. He earned his
PhD in June, 1920 for a thesis titled
Sopra gli spazi einsteiniani a gruppi continui di transformazione ("On
Einstein manifolds and groups of continuous transformations"). The thesis, which dealt with problems in
differential geometry related to
Albert Einstein's
theory of relativity, was defended in front of a jury of 11 mathematicians, including Levi-Civita,
Vito Volterra, and
Guido Castelnuovo. In the fall of 1920 he went to
Paris, where he gave talks in
Jacques Hadamard's seminar at the
Collège de France. While in Paris, he organized a seminar with other Romanian mathematicians, including Petre Sergescu, Șerban Gheorghiu, Alexandru Pantazi, and Șerban Coculescu. In 1922, he returned to
Bucharest, where he embarked on a 40-year-long university career. In 1924, he started teaching the first college-level probability theory course in Romania. From 1928 on, he was professor at the Faculty of Sciences of the
University of Bucharest, and was appointed full professor in 1931. In 1930, he organized the School of Statistics and established an Institute of Calculus, serving as its director for many years. In 1936 he entered the
Legionary Movement. The Romanian poet and mathematician
Ion Barbu (Dan Barbilian) was a close friend of his. Onicescu was an
invited speaker at the
International Congress of Mathematicians in 1928 at
Bologna and in 1936 at
Oslo. He was elected corresponding member of the Romanian Academy in 1933, and became titular member on February 4, 1965. He was in charge of the Probability Theory section of the
Institute of Mathematics of the Romanian Academy. He died in Bucharest on the eve of his 91st birthday, after a short illness. ==Legacy==