Ongaro was born to a middle-class family of Italian Argentines from the Friuli-Venezia Giulia region, in the Argentine seashore city of Mar del Plata in 1924.
Belonging to a CGT disoriented by the regime's surprise attack, Ongaro traveled to Cuba in early 1968, where, during a political conference, he met Argentine journalist and writer Rodolfo Walsh, with whom Ongaro flew to Madrid to introduce to the CGT's benefactor, exiled populist leader Juan Perón.