Together with his student, Gheorghe Mihoc, he is considered to be the founder of the Romanian school of probability theory and statistics.
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").
In the fall of 1920 he went to Paris, where he gave talks in Jacques Hadamard's seminar at the Collège de France.
Onicescu was one of the founders of the Balkan Union of Mathematicians (in 1934) and of the International Centre for Mechanical Sciences in Udine (in 1968).
The "Octav Onicescu" museum, founded in Botoșani in October 1995, houses furniture and memorabilia that belonged to him, including manuscripts, letters, diplomas, books, photographs, and military decorations.