An Olympic historian disagrees with a widely held consensus that American swimmer Michael Phelps is the greatest ever Olympian.

Phelps holds the record for career gold medals at 18, followed by Soviet gymnasts Larisa Latynina (nine) and Nikolai Andrianov (seven).

But as Bill Mallon points out, swimmers, gymnasts and runners, among others, benefit greatly from being more easily able to pile up medals in several events.

But discus throwers and pole vaulters, to use just a couple of examples, can win only one gold medal every four years.

Mallon, while freely acknowledging the greatness of Phelps, rates discus thrower Al Oerter the greatest Olympian.

Oerter, who died in 2007 at the age of 71, won four consecutive gold medals from 1956-68, a feat matched only by fellow American Carl Lewis (long jump, 1984-1996).

What sets Oerter apart, according to Mallon, is that he was an underdog in three of those Olympics, and a co-favourite in the other. He perfected the art of peaking at the right time.

"Oerter never won the U.S. Olympic trials," Mallon, a co-founder of the International Society of Olympic Historians, told Reuters. "Yet he won each gold with an Olympic record and a personal best."

Phelps will have two chances in Rio to join Oerter and Lewis on the four-peat list -- in the 100 metres butterfly and 200 individual medley.

Mallon rates Phelps and Lewis among those with claims to being the second-greatest Olympian, and says Usain Bolt will deserve consideration if the Jamaican sprinter can win the 100 and 200 metres and 4x100 relay for a third consecutive Games.

In individual events in Rio, Mallon says American gymnast Simone Biles, in the women's all-around, is perhaps the surest bet for gold, while the United States women's basketball team is the biggest lock for a team gold.

One other athlete Mallon will be watching closely is Japanese wrestler Saori Yoshida, who will shoot for a fourth consecutive gold.

Yoshida, 33, is not only unbeaten at the Olympics, but is a perfect 13-for-13 at world championships, stretching back to 2002.