Ontong was born in Johannesburg, and originally came to the UK from South Africa to pursue a football career; he had trials with Chelsea in the early 1970s.
From 1977 he was a regular in the Glamorgan side and would return to South Africa at the conclusion of each British summer to play more first-class cricket.
Following Mike Selvey's sudden retirement halfway into the 1984 season, Ontong became captain of Glamorgan and he finished the summer with a career best 74 wickets.
[1] He had arguably his best season in 1985 when he won the 'Cricket Society Wetherall Award' for the 'Leading All-Rounder in English First-Class Cricket' after making 1121 runs at 48.73 and taking 64 wickets at 27.76.
[4] As Ontong had met the qualifying requirements to represent England in international cricket, he was said to have been close to being picked as the country's Test all-rounder and was also in consideration for a spot in their 1987 World Cup squad.