Onyshko, 16, who burst on the international scene with bronze and silver medals on floor in her first World Cup meets last spring, earned gold this time around this time around with a score of 13.850 in the uneven bars final.
Jonna Adlerteg of Sweden won silver with 13.475 points and Rebeca Rodrigues de Andrade of Brazil took the bronze with 12.800.
Victoria Woo of La Prairie, Que., ran into problems and ended up seventh (10.200).
Onyshko, ranked second all around in Canada, will be looking to add to her growing medal collection in the beam and floor finals on Sunday while Woo competes again on floor.