onnis was born in giuliano di roma in italy, but moved to argentina before he was 3 and subsequently gained argentinian citizenship. beginning his football career as a youth player for club almagro, he made a successful transition to the senior team and subsequently earned a move to gimnasia la plata where he played alongside players like roberto zywica. he was the star striker for the team that finished third in 1970 which brought him to the attention of scouts from europe.