They are part of the 24-strong New Zealand team that will be matched against 120 of the best junior motocross riders from the United States, Australia and Europe.
King (15), a pupil at Kings High School, made his mark on the national scene when he won national titles at Mosgiel in 2007 and at Taupo in April this year.
He is the pick of the New Zealanders competing in the 85cc class but is a bit apprehensive about his prospects.
"It's the first time I've represented New Zealand and I feel a bit nervous," King told the Otago Daily Times.
Motocross is the family's sport.
King's father, Trevor, once finished fourth at the New Zealand championships.
"Dad bought me a bike when I was 5 and started to take me to national events by the time I was 9," he said.
King rides his bike five or six days each week and does strength work at a gymnasium.
The family has a 4ha lifestyle farm at Brighton.
King uses a track which runs around the house as a training circuit.
The riders he looks up to are New Zealand's Ben Townley (Taupo) and United States champion Ricky Carmichael.
Palmerston's Duncan (13), the only female rider to qualify for the world championships, will be riding a Yamaha bike and will be competing in the 85cc class.
Cromwell's O'Callaghan (16), who was originally named as a reserve, has been added to 125cc team.
International riders to watch this weekend include Glenn Coldenhoff (Netherlands) and Dylan Ferrandis (France), who won bronze medals in their classes at last year's world championships.
Another likely medallist is Davis Ivanovs (Latvia), who finished ninth at last year's junior world championships.
New Zealand is one of the world's strongest motocross countries.
Kiwi riders who have reached the podium at world championships include Townley, Josh Coppins, Daryl Hurley, Cody Cooper, Darryll and Shane King and Katherine Prumm.
Preliminary rounds of the junior championship will be held on Saturday, with the finals on Sunday.
