The unbeaten South Island team went on a 30-point second-half scoring spree to beat Napier Boys' High School 30-16 in windy conditions in Rotorua.
Otago Boys' will play St Kentigern College in Auckland this Saturday after the Auckland powerhouse beat Hamilton Boys' High School 29-13 in the other semifinal.
The Napier side, playing with a stiff northerly, dominated the first spell, scoring two penalties from the boot of centre Trinity Spooner and a try to second five Kalin Paewai.
Otago Boys' struggled to put phases together in the first half and was out-muscled at the breakdown by a determined Napier pack.
But captain Josh Renton was confident his Otago Boys' team could turn the deficit around after the break.
"They beat us up in the first half and that was something we needed to work on and I think we did, in the second half," Renton said.
"We were always confident and that wind was worth a lot of points. We just needed to hold on to the ball."
Otago Boys' was the first to score after the break through the boot of first-five Sam Ellis, who kicked with 100% accuracy for a tally of 15 points.
But Napier Boys' scored the first try of the second half, when winger Shae Tucker carried two would-be tacklers over the line to extend the lead to 16-3.
However, the momentum began to turn, thanks to the hard work of Otago Boys' lock Troy Callander and blindside flanker Josh Dickson. The rangy forwards combined well to set up and score the first of three tries in a sizzling 10-minute spell.
Otago Boys' had an attacking ruck on the left-hand side of the field, 5m out from Napier's line. Renton fired the ball right to Callander, who was waiting out in the backline. He stepped inside his man and offloaded to a charging Dickson, who scored under the post.
The next two Otago Boys' tries came from classic backline moves set up by Ellis. Both Ben Miller and Aleki Morris were given room outside their men and showed pace and power to get to the line and score.
