The 31-year-old South African opened with three birdies then added two more on the sixth and No. 18 to finish at 15-under 201.
I played solid and they are there to catch me, Otto said. I just tried to two putt every green and if a birdie came, it came.
Alastair Forsyth was second after shooting a 66.
The Scot pitched in from 8 yards out for an eagle-2 at No. 14 to go with eight birdies. However, he had four bogeys.
It could have been a really good day, Forsyth said.
Gary Clark of England was six back after shooting the days low round of 8-under 64.
Frances Michel Lorenzo-Vera and Fredrik Widmark of Sweden trailed by seven strokes at 208.