The Proteas' fielding standards have been a worry of late during the tour of England and in the recent home series against Bangladesh.
Ontong was still playing for the Cape Cobras this season and had been one of the best batsman in the four-day Sunfoil Series competition.
Cobras coach Prince has no doubts that Ontong, who has played two Test matches, will make a difference with the national side.
"At 37-years-old Justin is still possibly the best fielder in our team," Prince told IOL Sport.
"That says a lot not only about his great fielding ability, but also his high work ethic and standards he sets for himself. He will expect the same of the players he works with.
"He will add great value to the Proteas. Not only as a fielding coach, but having been a captain for so long, he also understands the game, especially limited-overs cricket and the dynamics related to what difference fielding can make to when games are decided by the smallest of margins."
Although Prince is losing one of his in-form and senior player's, he knows what a wonderful opportunity Ontong has with the Proteas.
"Justin was still making a massive contribution to the Cobras," said the Cobras coach.
"We were planning for the new year and he formed a big part of that, and if it was up to me, he would still be coming back in the 2018/19 season because I believed there were still lots of runs he would make for us.
"In regards to Justin, it is a wonderful opportunity and could even be the stepping stone to returning and becoming a head coach of a franchise team in the future."